Enable remotely `apt-get update`ing a box

This commit adds remote `apt-get update` functionality to
`mcv.remote.apt`.  This allow us to update apt information remotely
during the bootstrap phase, rather than needing to wait until the
local phase, in case we need up-to-date info during the earlier
phase for setting up the remote execution environment.
Merge pull request #48 from elliot42/master

Enable remotely `apt-get update`ing a box