Add RHEL7 to Red Hat family in pkg-map

The current "redhat" os family omits RHEL7. The rhel7 element has the
corresponding $DISTRO_NAME set in environment.d.  This patch adds rhel7
so a pkg-map json file using "redhat" for the os family will work the
same as rhel, centos and fedora.

Change-Id: If6b4cbba618167669a499e6121f234942ca9367a
Merge "Add RHEL7 to Red Hat family in pkg-map"