Add CentOS to is_fedora check

Match CentOS for centos7 nodes in "is_fedora" checks.  In the future
we may need a more fine-grained approach, but for now both can be
considered the same.

Change-Id: I7fd7933a94c3a7fa08f4f29a02fd36d09da21519
Merge "Add CentOS to is_fedora check"