Add create_iscsi_target stub to TargetAdmin

This should be defined in TargetAdmin, as it is assumed to be
present in all subclasses by callers.

IetAdm and LioAdm may need additional work to behave as desired
in all circumstances (such as volume extend), but this should
at least stabilize things in the meantime.

Closes-Bug: 1273480

Change-Id: Iac1381286ea07982e6db7f7c7abf3f9100e4be50
Merge "Add create_iscsi_target stub to TargetAdmin"