packaging: setup: storage: refactored plugin

Refactored storage plugin splitting NFS specific code in
a new nfs plugin.
Will allow to add iSCSI specific code in its own plugin.

Change-Id: Ided3e1f8e71abc2d0e3ec80e1cd2c2f3cca1f1df
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>