Sketch of implementation.
Merge branch 'master' into volume-yaml-271
fixes #271 with a few notes to take in to consideration:
1. some cleanup / refactoring should be done in flocker.node._config
   to eliminate duplication; a new issue for this makes sense
2. In reference to https://github.com/ClusterHQ/flocker/issues/271#issuecomment-49191998
   only ASCII paths supported at this time; a new issue for this should be raised
3. No docs on the new config item yet; to come in a separate commit
document optional volume parameter in application config
	modified:   configuration.rst
fix a typo, missing letter 'e'
addresses review comments
{"ports": [{"internal": 80, "external": 8080}]}
Long sentence into three shorter ones to ease readability.
stdlib imports at the top
Make it more clear that this is an error because of Flocker implementation limitations.
Update the test to agree with the new message.
Break long line.
Merge pull request #329 from ClusterHQ/volume-yaml-271

Support volume mountpoint configuration.  Fixes #271
Merge branch 'master' into reportstate-11

Conflicts:
	flocker/node/_config.py
	flocker/node/test/test_config.py