find_storage_pool_sources_as: Check for required utilities

In order to run the iscsi and logical tests, the 'tgtadm' and
'iscsiadm' utilities must be installed; otherwise, tests fail.
Add check and skip if not installed.
Add try/except for login_iscsi and create_vg failure

Add a try/except for login_iscsi or create_vg failure.  If the login_iscsi
fails, then we leave with the iscsiadm still logged in if the creation
of the vg fails.  If either fails, then use the exception to propagate
the error.
Merge pull request #33 from jferlan/storage_pool_sources_as

find_storage_pool_sources_as: Check for required utility