SanISCSIDriver SSH execution fixes.
SanISCSIDriver SSH execution fixes.

In ssh_execute methon in nova/utils there was a problem with cmd passed to it.
From SanISCSIDriver and all other places it is called from cmd is a string.
But in ssh_execute method cmd was processed as a list. This can make problems
with SanISCSIdriver using.

Also it may be useful to pass execute parameter to this SanISCSIDriver class
to have the opportunity of overriding _execute method (as it is in
VolumeDriver class, that is the root of this class hierarchy). That's why
special checking was added.

Fixes bug 1070489

Change-Id: I73f74f9d095a2c4316cab88148afe6a0bde44c5b

n0ano/gantt,n0ano/gantt