Fix exception handling in PureISCSIDriver

Fix exception handling in PureISCSIDriver

PureISCSIDriver had some error logging messages that make use of empty
replacement fields "{}" in strings on which the .format() method is
called, as allowed in Python 2.7+. Python 2.6 requires explicitly
naming or enumerating fields, i.e., "{}" must be replaced with "{0}".
This change fixes this so PureISCSIDriver is Python 2.6 compatible.

PureISCSIDriver.terminate_connection() also changed to catch
errors raised by _get_host_name(). Exception handling generally changed
to be more correct.

This change also adds testing of these code path to the unit tests,
to make sure it's actually correct.

Change-Id: I84179a8bc59dcf5593664ab11c90b07c451fd360
Closes-Bug: 1357004