[Feature] Add the possiblity to specify the used PostgreSQL version

[ZPascal]

Current state:
It's currently only possible to consume the default major version (currently PG 15) via the PostgreSQL Bosh release.

Improvement idea:
It should be possible to configure the used major version.

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this:

https://www.pivotaltracker.com/story/show/186216250

The labels on this github issue will be updated when the story is started.

[jpalermo]

@Jobsby is currently working on a solution for this

[ZPascal]

The problem was solved by PR #75. I am closing the case.