Checking for support of CentOS 6.4 in CI pool test

Basically this already works. Main issue is that uninstallation due to
the missing libcephfs1 package leaves packages around which conflict
with reinstallation of different version in the next test.
Same with the repository which needs to be set to absent otherwise the
newest release is always used.

Change-Id: I5a971104ff461d90217aaa33697191206135d22d
Closes-Bug: #1323655