(PUP-3069) Add an integration spec for default_manifest

This adds integration testing of the new default_manifest and
restrict_environment_manifest settings. It tests all the way from
Puppet initialization from actual puppet.conf files and
environment/manifest files through catalog compilation, simulating
Puppet master activity. Since these settings are purely server-side
constructs, in that they do not impact the agent beyond the contents of
the final catalog the agent receives, I believe this is sufficient to
validate them.