Fix several flake8 issues in the plugins/xenserver code

Due to some issues in the gate, several flake8 errors got merged
recently. This patch fixes the most recent issues found.

For the H304 relative import errors, we have to skip those because they
are put into /etc/xensource/scripts which is the current working
directory when running the plugin.

For the H231 incompatible python 3 'except x,y' construct, we have to
skip those because this code is written to run on python 2.4 and 'except
x as y' does not work with python 2.4.

Note that in cleaning up some of the H304 failures for relative imports,
I also re-arranged the imports to follow the hacking guide of doing
standard library packages first, then third party packages, and finally
nova-specific packages.

Closes-Bug: #1229753

Change-Id: I1c2211fd6a10d43d7e65cdb4e18530397788cf2c
Merge "Fix several flake8 issues in the plugins/xenserver code"