Doing proper mock cleanup

Now it checks if there are any mounted dirs inside the chroot and tries to
umount before removing. It will fail if unable to umount.

Also the cleanup has been put as post-build task, so it should always run.
Changed the logs tarball extension to more standard tgz

Change-Id: I20cb54fb968556f3d1bdcf3a9ab14e826c56b352
Signed-off-by: David Caro <dcaroest@redhat.com>