Push volumes from one node to another (fixes 16, 90)

This fixes #16, and also #90 so that the tests run on Buildbot.
Even though this is internal, we should probably document its use more completely.  People are going to find `flocker-volume` and poke at it.  Better they find sufficient information to understand how it works/is used than not.

In this case I think all that's missing is documentation for the two mandatory positional arguments.