Fix RDS enabled groups property

[markdboyd]

Changes proposed in this pull request:

Related to cloud-gov/opensearch-boshrelease#144

• Add docker-compose setup for running tests locally with PostgreSQL
• Add README docs on testing with PostgreSQL container
• Change type of EnabledCloudWatchLogGroupExports to pq.StringArray to allow saving strings to the database. This change does require that the broker's internal database use PostgreSQL, but every other type that I tried for this field had issues saving to the database. See How to define []string in Gorm Model go-gorm/gorm#4498
• Update test bootstrapping to allow using configured local PostgreSQL database
• Add test for setting EnabledCloudWatchLogGroupExports when modifying an RDS database

Things to check

• For any logging statements, is there any chance that they could be logging sensitive data?
• Are log statements using a logging library with a logging level set? Setting a logging level means that log statements "below" that level will not be written to the output. For example, if the logging level is set to INFO and debugging statements are written with log.debug or similar, then they won't be written to the otput, which can prevent unintentional leaks of sensitive data.

Security considerations

None, just fixing issues with persisting EnabledCloudWatchLogGroupExports to the database and adding necessary test setup to catch this issue locally

[JasonTheMain]

Looks good to me