Don't install percona-toolkit if distro is RHEL7

It's not available yet so this will break installs of keystone on
RHEL7 right now.  As noted, the check can be removed once
percona-toolkit is released for RHEL7.

Change-Id: I65b888e994b19bdfe0cd8e4686d27a34e5afa984
Merge "Don't install percona-toolkit if distro is RHEL7"