ceph-nfs: Add rpcbind to Ubuntu host bootstrap

* Ubuntu ships with nfs-ganesha 2.6.0, which requires to do an rpcbind
udp test on startup (was fixed later)
* Add rpcbind package to be installed by kolla-ansible bootstrap when
ceph_nfs is enabled
* Update Ceph deployment docs with a note

Change-Id: Ic19264191a0ed418fa959fdc122cef543446fbe5