Allow using a specific EMI and add an output option to write a file Nagios can parse

These commits have been used to help use the instancetest.py as part of a Nagios check on cloud availability.

First commit adds the option to specify a particular EMI with --emi <emi-id>. This is useful when running the basic instancetest.py on a regular basis as you may not want it to pick a random EMI. It defaults to the original behaviour of picking a random instance-store backed EMI.

The second commit adds an output option --nagios to output a PASS or a FAIL into /tmp/results. I parse this results file with a Nagios check script. I'm open to other ways to adapt that so it's useful in other tests. Perhaps using the --xml output and using something to parse the junit style xml output?