feature: load bundle from store

[kwmonroe]

It would be great to do this in an amulet test:

cls.d = amulet.Deployment(series='trusty')
cls.d.load_bundle_from_store('cs:hadoop-processing')

There's a load and load_bundle_file, which is great:

https://github.com/juju/amulet/blob/master/amulet/deployer.py#L97

One could wget the bundle and use one of those methods on the fetched yaml, but I would like to be able to load a bundle directly from the store in my amulet test.