Parse out '@' in volume['host'] to do discovery

Parse out '@' in volume['host'] to do discovery

The backup method of getting iscsi info is to use
iscsiadm discovery, however currently that method
just uses volume['host'] which in the case of
multi-backend will use "host@backend-name".

This will cause the discovery to fail of course, so
this change just parses out the '@' symbol if it's present
and avoids the problem in the first place.

This also beefs up the error logging and exception catching
a bit.

Parsing out the '@' symbol all the time should be safe as
the accepted valid chars for hostnames are digits, a-z and
hyphens.

Change-Id: Ic45a38bf4c56a4aec6847ab0d29e3b41d35bd3d2
Closes-Bug: #1250673