[puppet_agent] Allow disabling repository configuration

Currently the puppet_agent module manages configuration of repositories providing the puppet-agent package. This makes it incompatible with users utilizing existing systems management tools and internally hosted repositories to ensure agent upgrades (such as spacewalk/satellite).

In scope for agent team:

Write acceptance test to install the agent, remove our repo configuration (if present), verify that module does not re-add the repo if set to false, does add the repo if set to true.