[stable/postgresql] Fixed service name to be able to override it

[rodcloutier]

Changed the name of the service to not use the name of the release.

This currently prevents the chart from being used as requirements since addressing the service from another chart requires to know the name of the future release.

Given an umbrella chart that would provide both postgresql and another chart that would use postgresql:

# requirement.yaml
dependencies:
- name: postgresql
  version: 0.8.1
  repository: @stable
- name: my-chart
  version: 0.1.0
  repository: @local

If my-chart has the following required values (to find the postgresql service):

# values.yaml

# This currently cannot be achieved since we would need to know ahead
# of time the name of the release
my-chart:
  pgdb_service: <name of the service>

The proposed changed would allow:

# values.yaml
postgresql:
  nameOverride: my-postgresql
my-chart:
  pgdb_service: my-postgresql

[k8s-ci-robot]

Hi @rodcloutier. Thanks for your PR.

I'm waiting for a kubernetes member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[mgoodness]

After #1777 you can refer to the postgres service using {{ template "postgresql.fullname" . }}.

Closing this PR, but feel free to reopen if the above doesn't work for you.