CLOUDSTACK-2685
Fix snapshot GC test

Changes to object store removes SecondaryStorage host from the host
table. This affects the test where we identify the secondary storage nfs
host to login to where we verify if the snapshot is present on the
physical resource.

* added base classes for imagestore/stagingstore operations
* fixed the test to check only if NFS is used as backing store

TODO: Fix for s3 based storage layer

Signed-off-by: Prasanna Santhanam <tsp@apache.org>
remove local path in cloud-set-guest-password script

Signed-off-by: Prasanna Santhanam <tsp@apache.org>
Fix snapshot limit tests

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