Fixed subscription-manager registration

Optional channel should not be enabled by default and relevant OS channel
should be enabled by default. This patch is implementing those features.

Priority of plugin serverprep has been changed since we need repositories
before we touch any other packstack plugins.

Change-Id: Id5a8e89c7bb817c99b398ef0053e1c35a7a21cab
Fixes: rhbz#1093482
Merge "Fixed subscription-manager registration"