Don't use _execute directly in brick/iscsi

The brick/iscsi module has a run helper that
should be used for executing commands.  There
are a number of inconsistencies where _execute
is called directly.

This patch moves everythign to use the run method
to keep things consistent and also to fix up some
potential issues with variables becoming corrupt
under heavy load.

Change-Id: Idfc183f2ed1ad73b64fc893efcc07972c95926c6
Merge "Don't use _execute directly in brick/iscsi"