Fix parsing sloppiness from iscsiadm discover

As reported by a user, iscsiadm might spew some other random gorp when
it starts up, and if it does, nova explodes poorly. Put a modicum of
sanity testing into the parsing of the output to make this less
likely.

Change-Id: Iaf55438bb3ec412187158e80f9fbc1d8e7071207
Closes-Bug: #1308649
Merge "Fix parsing sloppiness from iscsiadm discover"