Add env var support for ssl mode

[benwalk]

Description

SSL mode for the database connection string was not configurable. This change allows configuration of this via env var DB_SSL_MODE.

Motivation and Context

Enabling encrypted connections to backend databases should be allowed.

This issue is filed upstream at cloudfoundry#4434.

How Has This Been Tested?

The existing tests validate TLS connection strings work.

The value of the supplied mode is already validated here.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• Docs update
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have followed the guidelines in CONTRIBUTING.md, including the required formatting of the commit message