Make default acquia-pipelines.yml configuration distributable

[grasmash]

At present, when BLT makes a change to its default acquia-pipelines.yml, a BLT user must delete their local acquia-pipelines.yml file and re-initialize Pipelines support.

We should create a mechanism that allows these changes to be rolled out without destroying customizations in user's local acquia-pipelines.yml files.

The suggested approach is to used "partials" to import portions of BLT's default acquia-pipelines.yml files rather than dictating the contents of the entire file.

Blocked by MS-1873.

[grasmash]

This is being addressed by #1040.