Merge lp://staging/~sinzui/juju-ci-tools/run-instance-with-series into lp://staging/juju-ci-tools

Create manual instances that match the series under test.

substrate.run_instances() only makes precise instances. The get_ami() is always passed "precise". This branch adds a series argument to run_instances(). There was only one call site, so I chose to make this a required argument. All the caller may have been given a series of None, we require a guard to assume precise. While every caller of boot_context should be passing series, we really don't know if every caller is explicitly passing series as they should.