[vagrant] Allow tasks on all/subset of vms - #177

This is updated now. I suspect that with "vagrant.settings()" probably doesn't handle this either, but I honestly don't know how to use that, and I couldn't find any use of it in the tests.

list_vms is broken here for me, using Vagrant 1.3.5.

Here is an example of vagrant status output:

$ vagrant status
Current machine states:

default                   running (vmware_fusion)

The VM is running. To stop this VM, you can run `vagrant halt` to
shut it down, or you can run `vagrant suspend` to simply suspend
the virtual machine. In either case, to restart it again, run
`vagrant up`.

So if we grep for "running", we get:

$ vagrant status | grep running
default                   running (vmware_fusion)
The VM is running. To stop this VM, you can run `vagrant halt` to

Vagrant 1.4 adds machine-readable output. Maybe we should require that.

Now that I've stepped away from this for a while, I'm coming around to this idea. I rather like the ability to specify several specific vagrant machines from the command line.

Regarding @ronnix's point about --machine-readable output of vagrant 1.4, it sounds like an awesome idea. The only downside is that the documentation also makes it sound like this feature is likely to change quite a bit. What if we instead changed the grep command to something like this etactica#1

I still personally won't use the "all running by default", mostly because I tend to have one development environment with virtualbox and one production environment with rackspace running at all times. More often than not, I only want to run fab on one of those environments (either of which might consist of several vagrant VMs). So I'll still probably stick with the fab specify_hosts some_task pattern but that doesn't mean it won't be useful for others.