Add NFS storage domain functional tests

Test creating NFS storage domain, pool, image, volume.
Test creating VM with the NFS storage.

How to verify:
1. build and install VDSM rpms, including vdsm-tests
2. start VDSM service
3. cd /usr/share/vdsm/tests
4. ./run_tests.sh -m 'nfs' functional/xmlrpcTests.py

Change-Id: Ic9c24369af552b5d58e0f5704075d6f351792775
Signed-off-by: Zhou Zheng Sheng <zhshzhou@linux.vnet.ibm.com>
Reviewed-on: http://gerrit.ovirt.org/13105
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>