CA-115273 : Block XenCert MetaData tests on NFS and HBA

Currently MetaData tests could be run although they 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

Signed-off-by: Sugandha Aggarwal <sugandha.aggarwal@citrix.com>
Reviewed-by: Vineeth Thampi Raveendran <vineeth.thampi@citrix.com>

GitHub: closes #13 on xenserver/xencert