Allow use of in-cluster kube-client for test-command

[janpieper-gcx]

This PR fixes the issue described in #288.

With Kubernetes

For the following cases, I used the following registries.conf:

registries:
  - name: myfancyregistry
    api_url: https://myfancyregistry.azurecr.io
    prefix: myfancyregistry.azurecr.io
    ping: yes
    credentials: secret:argocd/registry#auth

In-cluster kube client **NEW**

Success

$ argocd-image-updater --registries-conf registries.conf test myfancyregistry.azurecr.io/example
DEBU[0000] Creating in-cluster Kubernetes client        
INFO[0000] getting image                                 image_name=example registry=myfancyregistry.azurecr.io
DEBU[0000] rate limit for https://myfancyregistry.azurecr.io is 2147483647 
INFO[0000] Loaded 1 registry configurations from registries.yaml 
INFO[0000] Fetching available tags and metadata from registry  image_name=example
INFO[0001] Found 2 tags in registry                      image_name=example
DEBU[0001] found 2 from 2 tags eligible for consideration  image=myfancyregistry.azurecr.io/example
INFO[0001] latest image according to constraint is myfancyregistry.azurecr.io/example:v0.0.1

Failure

$ argocd-image-updater --registries-conf registries.conf test myfancyregistry.azurecr.io/example
DEBU[0000] Creating in-cluster Kubernetes client        
INFO[0000] getting image                                 image_name=example registry=myfancyregistry.azurecr.io
DEBU[0000] rate limit for https://myfancyregistry.azurecr.io is 2147483647 
INFO[0000] Loaded 1 registry configurations from registries.yaml 
FATA[0000] could not set registry credentials: could not fetch secret 'registry' from namespace 'argocd' (field: 'auth'): secrets "registry" not found

Local kubeconfig

Success

$ argocd-image-updater --registries-conf registries.conf test --kubeconfig ~/.kube/config myfancyregistry.azurecr.io/example
DEBU[0000] Creating Kubernetes client from /home/jan/.kube/config 
INFO[0000] getting image                                 image_name=example registry=myfancyregistry.azurecr.io
DEBU[0000] rate limit for https://myfancyregistry.azurecr.io is 2147483647 
INFO[0000] Loaded 1 registry configurations from registries.yaml 
INFO[0000] Fetching available tags and metadata from registry  image_name=example
INFO[0001] Found 2 tags in registry                      image_name=example
DEBU[0001] found 2 from 2 tags eligible for consideration  image=myfancyregistry.azurecr.io/example
INFO[0001] latest image according to constraint is myfancyregistry.azurecr.io/example:v0.0.1

Failure

$ argocd-image-updater --registries-conf registries.conf test --kubeconfig ~/.kube/config myfancyregistry.azurecr.io/example
DEBU[0000] Creating Kubernetes client from /home/jan/.kube/config 
INFO[0000] getting image                                 image_name=example registry=myfancyregistry.azurecr.io
DEBU[0000] rate limit for https://myfancyregistry.azurecr.io is 2147483647 
INFO[0000] Loaded 1 registry configurations from registries.yaml 
FATA[0000] could not set registry credentials: could not fetch secret 'registry' from namespace 'argocd' (field: 'auth'): secrets "registry" not found

Without Kubernetes

As we're now using --disable-kubernetes, we cannot use a kubernetes secrets for the credentials-option anymore. In this case, I am using an ENV-variable:

registries:
  - name: myfancyregistry
    api_url: https://myfancyregistry.azurecr.io
    prefix: myfancyregistry.azurecr.io
    ping: yes
    credentials: env:REGISTRY_SECRET

Success

$ REGISTRY_SECRET=user:password argocd-image-updater --registries-conf registries.conf test --disable-kubernetes 
INFO[0000] getting image                                 image_name=example registry=myfancyregistry.azurecr.io
DEBU[0000] rate limit for https://myfancyregistry.azurecr.io is 2147483647 
INFO[0000] Loaded 1 registry configurations from registries.yaml 
INFO[0000] Fetching available tags and metadata from registry  image_name=example
INFO[0000] Found 2 tags in registry                      image_name=example
DEBU[0000] found 2 from 2 tags eligible for consideration  image=myfancyregistry.azurecr.io/example
INFO[0000] latest image according to constraint is myfancyregistry.azurecr.io/example:v0.0.1

Failure

$ REGISTRY_SECRET=user:password argocd-image-updater --registries-conf registries.conf test --disable-kubernetes ~/.kube/config myfancyregistry.azurecr.io/example
INFO[0000] getting image                                 image_name=example registry=myfancyregistry.azurecr.io
DEBU[0000] rate limit for https://myfancyregistry.azurecr.io is 2147483647 
INFO[0000] Loaded 1 registry configurations from registries.yaml 
WARN[0000] cannot use K8s credentials without Kubernetes client  registry="https://myfancyregistry.azurecr.io"
FATA[0000] could not set registry credentials: could not fetch image tags

[jannfis]

LGTM, thanks @janpieper-gcx

[jannfis]

Closes #288