(maint) Updating site-defaults should require mock

Previously, the tmpfs_plugin class would attempt to modify
site-defaults.cfg before it was available. This is because it is
supplied by the mock package, which won't be on a bare system. This
commit updates these ini_setting resources to require the mock package
before updating site-defaults.cfg.
(maint) Require mock before laying down mock configs

Previously we were trying to template files into /etc/mock, a directory
that won't exist until the mock package is installed. This commit
updates the class resources to require mock before proceeding with
laying down the mock configs.
Merge pull request #81 from haus/maint/master/require_mock

(maint) Require mock before doing things in /etc/mock