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
Merge "allow error code 21 when checking iscsi session before login"