Allow setting the apt mirror in config.yaml

Allows values of Yes, No, or an explicit mirror. Yes is the default
behaviour, and uses autodetection to grab the mirror.
Revert "Prevent the mirrors from being used. Related: #84"

This reverts commit 760d5eee92a475991ccdff29009941fe927521aa.

Re-enables mirror setting, now that we can safely use it on all systems.
Correctly handle apt_mirrors set to no
Ensure Yes and mirror domains work correctly
Merge pull request #85 from Chassis/mirror-configuration

Allow configuring apt mirrors Props @rmccue