[vagrant] Allow tasks on all/subset of vms

This works by relying on the fact that vagrant's ssh-config for multiple
machines differs only in the host string.

Behaviour:

fab vagrant task          # runs on all running vagrant vms
fab vagrant:name1         # runs on only name1 machine
fab vagrant:"name1;name3" # run on name1 and name3

The ; is required so that fabric doens't split it into multiple
positional arguments, as noted at
http://docs.fabfile.org/en/1.7/usage/fab.html#per-task-arguments

This fixes issue #176