allow error code 21 when checking iscsi session before login

On a node with no iscsi established sessions running (e.g. a new
node) 'iscsiadm -m session' may return error code 21 signifying
that there aren't any sessions, etc. to execute on. In this case
it's perfectly valid and the login should still proceed.

Change-Id: I1cee2a1aa55e44b9a66b8a1809d75c4cbd703777
update to match latest nova method signatures

Nova made backwards-incompatible changes in how it
calls these methods in the driver.  Altered the
method signatures to match the new expectation.

Change-Id: I02182dae9159bcac671899df846e50b301f16003
Merge "allow error code 21 when checking iscsi session before login"