Pre-install hgtools for RHEL6

See the inline comment; pre-installing this package works around [1]

[1] https://bugzilla.redhat.com/show_bug.cgi?id=924038

Change-Id: Id021b44ac9d8aa28286828ccfd6515b7850bad16
Merge "Pre-install hgtools for RHEL6"
Merge branch 'master' of github.com:openstack-dev/devstack