Upgrade uses nc, but doesn't enforce its presence.
This makes sure nc is installed.
On top of it, the filenames used are ubuntu ones,
so this moves to use a var to find which filename
should be used for centos.
See also the issue here:
http://logs.openstack.org/periodic/periodic-openstack-ansible-
upgrade-ceph-master-centos-7/d2fc540/
console.html#_2017-05-30_08_23_40_475244
Change-Id: I3ee2dfd6e435fb4d92a57e2debf7f001f7c03f98