CA-115273 : XenCert MetaData tests can be run for NFS and HBA although t...

...hey are not implemented

 The metadata test needs to be run conditionally based on storage type:
 It shouldn't run on NFS SR type, because the SR metadata concept doesn't apply to file-based SRs
 It shouldn't run on HBA SR type, because the testcase wasn't implemented for the SR type (unfortunately)
 It should run on ISCSI and ISL though