teuthology-lock: add centos 7.2 to downburst default table
teuthology-lock: add centos 7.2 to downburst default table

When downburst is unavailable, teuthology-lock reverts to default_table and
fails because the only supported CentOS version, 7.2., is missing.

Signed-off-by: Nathan Cutler <f9d8c66a4e898e67db786769b5b347b640e1a9a6@suse.com>

ceph/teuthology,SUSE/teuthology,dmick/teuthology,dmick/teuthology,ktdreyer/teuthology,dmick/teuthology,SUSE/teuthology,ktdreyer/teuthology,SUSE/teuthology,ceph/teuthology