Document passing private key to mcv.remote.connection

This commit adds a comment explaining how to pass a private key file
to mcv.remote.connection (thus/the same way as passing it to the
underlying Paramiko).

This is helpful when you want to specify a private key.  This is
particularly useful/necessary for connecting to a Vagrant instance.
Merge pull request #11 from elliot42/private-key

Document passing private key to mcv.remote.connection
Make `filesystem.partition` idempotent

This commit makes the `partition()` operation idempotent by checking
whether the requested device is already partitioned and no-op'ing if
so.

Right now this works via a hack (assume that partitioning "/dev/FOO"
produces "/dev/FOO1"), but the alternatives would take some work/time
that we don't have right now.

In the future there may be cleaner alternatives.