Fix helm chart so it is compatible with Go 1.14

[jon-ruckwood]

Additionally, add 'cluster' to fix type error.

See helm/helm#7711 (comment)

Fixes #1664

[jon-ruckwood]

Lint output:

$ helm lint  ./helm/camel-k                                                                             
engine.go:159: [INFO] Missing required value: Field "platform.build.registry.address" is required when not running on OpenShift (set "platform.cluster=OpenShift" if you're using OpenShift instead)!
==> Linting ./helm/camel-k

1 chart(s) linted, 0 chart(s) failed

Install locally:

$ helm install --generate-name --set platform.build.registry.address=http://localhost:5000/ ./helm/camel-k
NAME: camel-k-1611242562
LAST DEPLOYED: Thu Jan 21 15:22:44 2021
NAMESPACE: sandbox
STATUS: deployed
REVISION: 1
TEST SUITE: None
NOTES:
Connect the world with Camel K on Kubernetes!

[astefanutti]

Thanks!