added support for ispersistent parameter for networkoffering

Signed-off-by: Prasanna Santhanam <tsp@apache.org>
CLOUDSTACK-4647: Fixed snapshot_gc test case and common util function is_snapshot_on_nfs

Signed-off-by: Prasanna Santhanam <tsp@apache.org>
CLOUDSTACK-4780: Fixed BVT test case TestSnapshotRootDisk.test_01_snapshot_root_disk

Signed-off-by: Prasanna Santhanam <tsp@apache.org>
CLOUDSTACK-4256 - Fixed shared network test cases

Signed-off-by: Prasanna Santhanam <tsp@apache.org>