Two phase handoff - fixes 373

Fixes #373.

DO NOT MERGE UNTIL AFTER #46. Two-phased push without incremental push will result in copying all of the data twice.
Maybe use a more descriptive name for this factory function.

Maybe add some documentation explaining what "default production" / "standard" means ie 
 * default ssh port 22
 * root username
 * SSH private key from a standard flocker path.