Add Fedora 18+ and Kerberized NFS4 support

Very nice module, thank you!  These changes allow the module to _really_ meet all our site's NFS needs.

New:
* Remove support for "redhat" in favour of "rhel/centos" and "fedora" in order to support for systemd and new NFS service names on Fedora
* Manage rpc.gssd, rpc.svcgssd on Red Hat systems (running/enabled/flags stored in /etc/sysconfig/nfs)
* Manage the Kerberos realm in /etc/idmapd.conf
* Manage /etc/idmapd.conf with ERB templates instead of augeas