Activate pep8 check that _ is imported

Removed the specification in tox.ini that _ is a builtin so that
it will no longer assume that _ does not need to be imported.

The correct import is:

    from cinder.i18n import _

Activating this check did not flag any violations in cinder.

Change-Id: Ifc789514d2462c059eb9f003fac288e751331ded
Retry remove iscsi target

There is a race in the gate when removing an iscsi
target in the remove export flow where the target
is still active. It's about a 75% failure rate so
simply adding a configurable retry on the delete
call should clean that up.

Change-Id: I8531652e8b80f1c65c9942e109dea01fda5674e0
Closes-Bug: #1363258
Fix the LV NotFound situation for thin-type LVM

If the logical volume is not found, LVM displays on the error output
that the volumes could not be found. So here, we filter on this very
specific situation, and let all the other cases go through the stack.

Added a test for this new code path, which raises an exception of the
proper type to be caught by the new code.

Change-Id: I703af8ccd87c6332d9f88eff63fcf26ebed234f4
Closes-Bug: #1390081
Merge "Retry remove iscsi target"
Merge "Activate pep8 check that _ is imported"