validation for NFS URI (bug #1170700)

Added NFS uri validation for cinder and glance.

https://bugzilla.redhat.com/show_bug.cgi?id=1170700

Validates format of <server IP or hostname/fqdn>:<local path>

The first element must be an IP address or valid hostname/fqdn.
For the local path element, the only validation is that it's
not empty.
Merge pull request #386 from sseago/nfs_uri_validation

validation for NFS URI (bug #1170700)