Introduce getMountPoint for NfsStorageDomain

Move the code to generate the mount point into a seperate
function, so that classes reusing NfsStorageDomain can
override the function to present their own mount point path.

Change-Id: I80f98eddcc31c1f01f42af838ecc483abb955f41
Signed-off-by: Deepak C Shetty <deepakcs@linux.vnet.ibm.com>
Reviewed-on: http://gerrit.ovirt.org/8085
Reviewed-by: Dan Kenigsberg <danken@redhat.com>