Properly support `main:` and provide a helper for this

What does this MR do?

This is:

• Updates code to support main:
• Non-breaking change
• Adds a helper to check if given name is a main_database, this is to be used instead of .primary?
• This adapts config/database.yml.postgres to ensure that :main is reflected in rspecs (this is temporarily removed to ensure that application continues working properly till we update Omnibus/CNG).

Resolves:

• #334776 (comment 614031940)

Does this MR meet the acceptance criteria?

Conformity

• [-] I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• [-] I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have self-reviewed this MR per code review guidelines.
• [-] This change is backwards compatible across updates, or this does not apply.