propellor orchestration

With the disclamer that I don't really know much about orchestration, I have added support for something resembling it to Propellor.

Until now, when using propellor to manage a bunch of hosts, you updated them one at a time by running propellor --spin $somehost, or maybe you set up a central git repository, and a cron job to run propellor on each host, pulling changes from git.

I like both of these ways to use propellor, but they only go so far...

Perhaps you have a lot of hosts, and would like to run propellor on them all concurrently.
```
  master = host "master.example.com"
      & concurrently conducts alotofhosts
```
Perhaps you want to run propellor on your dns server last, so when you add a new webserver host, it gets set up and working before the dns is updated to point to it.
```
  master = host "master.example.com"
      & conducts webservers
          `before` conducts dnsserver
```

Perhaps you have something more complex, with multiple subnets that propellor can run in concurrently, finishing up by updating that dnsserver.

  master = host "master.example.com"
      & concurrently conducts [sub1, sub2]
          `before` conducts dnsserver

  sub1 = "master.subnet1.example.com"
      & concurrently conducts webservers
      & conducts loadbalancers

  sub2 = "master.subnet2.example.com"
      & conducts dockerservers

Perhaps you need to first run some command that creates a VPS host, and then want to run propellor on that host to set it up.
```
  vpscreate h = cmdProperty "vpscreate" [hostName h]
      `before` conducts h
```

All those scenarios are supported by propellor now!

Well, I haven't actually implemented concurrently yet, but the point is that the conducts property can be used with any of propellor's property combinators, like before etc, to express all kinds of scenarios.

The conducts property works in combination with an orchestrate function to set up all the necessary stuff to let one host ssh into another and run propellor there.

main = defaultMain (orchestrate hosts)

hosts = 
    [ master
    , webservers 
    , ...
    ]

The orchestrate function does a bunch of stuff:

Builds up a graph of what conducts what.
Removes any cycles that might have snuck in by accident, before they cause foot shooting.
Arranges for the ssh keys to be accepted as necessary.
Note that you you need to add ssh key properties to all relevant hosts so it knows what keys to trust.
Arranges for the private data of a host to be provided to the hosts that conduct it, so they can pass it along.

I've very pleased that I was able to add the Propellor.Property.Conductor module implementing this with only a tiny change to the rest of propellor. Almost everything needed to implement it was there in propellor's infrastructure already.

Also kind of cool that it only needed 13 lines of imperative code, the other several hundred lines of the implementation being all pure code.