CA-148436: Fix xencert metadata tests for ISCSI SR

Changes part of this commit:
* SR ATTACH (Switch to a different method to 'Create_SR'
* SR SCAN (Blacklist 'metadata_of_pool' & 'snapshot_time')
* SR PROBE (Log successful parameter validation)

Additionally, whitespace errors were corrected in the affected methods.

Signed-off-by: Siddharth Vinothkumar <siddharth.vinothkumar@citrix.com>
Acked-by: Chandrika Srinivasan <chandrika.srinivasan@citrix.com>

GitHub: closes #31 on xenserver/xencert
CA-148436: Removed SR probing from iSCSI tests

Probing is broken for reasons yet to investigate further.
Removing for the time being

Signed-off-by: Germano Percossi <germano.percossi@citrix.com>
Acked-by: Chandrika Srinivasan <chandrika.srinivasan@citrix.com>

GitHub: closes #33 on xenserver/xencert