Add ability to use CentOS local mirror

We could specify ubuntu and fedora local mirror for image build,
now we can use local mirrors for all type of images.

Yum-mirror element was divided in two elements: fedora-mirror
and centos-mirror

Change-Id: Id5016d8a7e982010b6683c038afa21e639ef4ac0
Merge "Add ability to use CentOS local mirror"