Merge lp://staging/~sinzui/juju-ci-tools/azure-arm-substrate into lp://staging/juju-ci-tools

Add support to delete a single instance in Azure ARM.

This branch adds delete_instance() and the helper find_vm_instance() to the winazurearm.py lib.
I The branch is long so I am not integrating this into substrate.py just yet.

This branch contains a few drive by fixes and a refactoring of the tests.

There are cases where the poller can be None when deleting, but after I merged my branch, I saw it was
often None. Oops.
1. Deleting a resource group did not return the poller.
   This was broken just before I merged my first branch.
2. A call to log.debug() didn't pass the name of the group that had the None poller.

I didn't like mocking the call to ResourceGroupDetails.delete. Since the client is mocked,
I decided to mock the client delete() methods so that I could check that the call args matched
the documentation. i like this refactoring, but it changed most of the tests in the module.

There is nothing wrong with delete, but since the azure lib is incomplete.
I want to clearly know what we think the call args are in case it changes.

My next branch will update substrate.py to call delete_instance().