Addition of Pre/Post start hooks for postgres container.

[flamingdumpster]

Checklist:

• [x ] Have you added an explanation of what your changes do and why you'd like them to be included?
• [x ] Have you updated or added documentation for the change, as applicable?
• Have you tested your changes on all related environments with successful results, as applicable?

Type of Changes:

• Bug fix (non-breaking change which fixes an issue)
• [ x] New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

What is the current behavior? (link to any open issues here)

What is the new behavior (if this is a feature change)?
Adds the ability to provide a pre-start and post-start shell script that will execute before or after, respectively, the database is started within the container.

Other information: