Add centos/redhat 7 support

This is something of a workaround for value_for_platform(), because
the version scheme between RHEL7 and CentOS7 differ. This enburdens
us to make quarterly changes to keep this cookbook functioning if
we are explicit with versions.
By making systemd 'default', we can take advantage of the regular
version scheme of 6.x to correctly identify them as 'sysv'. This
commit presumes we are not using el5.