[FLOC-1023] Allow an Application restart_policy to be configured in application.yml

Fixes https://clusterhq.atlassian.net/browse/FLOC-1023
And indeed when I tried live test running `flocker-deploy` with "always" policy I got an exception. This suggests some missing testing, as well.

```
Traceback (most recent call last):
  File "/opt/flocker/lib/python2.7/site-packages/twisted/internet/defer.py", line 577, in _runCallbacks
    current.result = callback(current.result, *args, **kw)
  File "/opt/flocker/lib/python2.7/site-packages/twisted/internet/defer.py", line 839, in _cbDeferred
    self.callback(self.resultList)
  File "/opt/flocker/lib/python2.7/site-packages/twisted/internet/defer.py", line 382, in callback
    self._startRunCallbacks(result)
  File "/opt/flocker/lib/python2.7/site-packages/twisted/internet/defer.py", line 490, in _startRunCallbacks
    self._runCallbacks()
--- <exception caught here> ---
  File "/opt/flocker/lib/python2.7/site-packages/twisted/internet/defer.py", line 577, in _runCallbacks
    current.result = callback(current.result, *args, **kw)
  File "/opt/flocker/lib/python2.7/site-packages/flocker/node/_config.py", line 1274, in marshal_configuration
    result[application.name] = converter.convert()
  File "/opt/flocker/lib/python2.7/site-packages/flocker/node/_config.py", line 183, in convert
    restart_policy = self.convert_restart_policy()
  File "/opt/flocker/lib/python2.7/site-packages/flocker/node/_config.py", line 198, in convert_restart_policy
    maximum_retry_count = policy.maximum_retry_count
exceptions.AttributeError: 'RestartAlways' object has no attribute 'maximum_retry_count'
```