Medium: nfsserver: do redhat specific stuff on redhat

The /etc/sysconfig/nfs editing code introduced in 2014 is
specific to RH based linux. At best it has no effect on other
distributions and at worst it can break the RA operation and/or
nfs configuration.

There is rudimentary support for distribution detection in
ocf-distro. It should serve well for this purpose.

NB: ocf-distro is intended to be used just here and just for this
purpose and not to be further developed.

Distribution-specific code is _not_ encouraged.