Add optional ability to install postgres chart 8.x

First step in moving off the 0.7.1 version of the chart.

Related issues:

• gitlab-org/gitlab#32720 (closed)
• gitlab-org/charts/auto-deploy-app#28 (closed)

The major differences jumping from 0.7.1 to 8.X:

• Everything in the postgresql chart is renamed from postgres to postgresql
• Deployment for the postgres server is now a StatefulSet
• Use Bitnami managed docker image by default (which does not have root)

Development

• Install postgresql chart 8.2.1 if POSTGRESQL_ENABLED env var is set
  • unit test: can install postgresql chart 8.2.1
  • integration test: postgresql chart 8.2.1 is installed if POSTGRESQL_ENABLED env var is set
• send different application.database_url to auto-deploy-app if POSTGRESQL_ENABLED env var is set
  • unit test: test correct database url depending on whether POSTGRESQL_ENABLED env var is set
  • integration test: test application pod can connect to database url with psql when POSTGRESQL_ENABLED env var is set/not set.