Add feature to set test iteration count from commandline

This is a kludge, but I think doing this properly basically requires
implementing our own Nose test runner, or even our own test-running
framework entire. In the short term we really need this feature.

nosetests doesn't provide a way for the user to pass command-line
arguments to the test. To get around this, this commit adds a
parameter to lisa-test that sets an environment variable, which
LisaTest then reads to set or override the iterations count.