[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:name3,name1   # run on name3 and name1

Hosts are kept in the order they are entered, if given.