add service linking for deploio apps

[gajicdev]

Related to #1591.

Summary

• Add --service flag to nctl create application and nctl update application for linking On-Demand Services (KeyValueStore, MySQL, PostgreSQL) to Deploio apps with automatic credential injection
• Add --delete-service flag to nctl update application for removing service references
• Show linked services in nctl get application table output

Examples

Create an application with service references

$ nctl create application my-app \
    --git-url=https://github.com/example/app.git \
    --service cache=keyvaluestore/my-kvs \
    --service db=mysql/my-db

Add a service reference to an existing application

$ nctl update application my-app --service cache=keyvaluestore/prod-kvs

Remove a service reference

$ nctl update application my-app --delete-service cache

View linked services

$ nctl get application my-app

NAME      REPLICAS  WORKERJOBS  SCHEDULEDJOBS  SERVICES                                    HOSTS               UNVERIFIEDHOSTS
my-app    2         0           0              cache=keyvaluestore/my-kvs,db=mysql/my-db    my-app.deploio.app  none

Flag help

$ nctl create application --help
...
      --service=KEY=VALUE;...      Service reference in the form
                                   name=kind/target-name. Credentials will
                                   be automatically injected as environment
                                   variables.

$ nctl update application --help
...
      --service=KEY=VALUE;...      Service reference to add/update in the form
                                   name=kind/target-name.
      --delete-service=DELETE-SERVICE,...
                                   Service reference names to remove.

Test plan

• Unit tests for ParseServiceTarget, ServicesFromMap, UpdateServices (internal/application/service_test.go)
• Integration test: create application with --service flag (create/application_test.go)
• Integration test: update application to add service (update/application_test.go)
• Integration test: update application to delete service (update/application_test.go)
• All existing tests continue to pass (go test ./...)
• Manual: nctl create application with --service flag against a real cluster
• Manual: nctl update application with --service and --delete-service against a real cluster
• Manual: nctl get application shows SERVICES column

[thirdeyenick]

👍