Comment 5 for bug 1735691

For sponsor - there is already an update for percona-xtradb-cluster-5.6 in the UNAPPROVED queue for artful; this update forces use of gcc-6 for compilation as gcc-7 resulted in a built that segfaulted continually. The security updates have been made ontop of this change, and percona-galera-6 has been moved to the same model as part of the security update for artful.