Flocker config volume size floc 979

Fixes FLOC-979
I feel like this `KeyError` is too broad -it might catch things caused by bugs in the parser code and mistakenly and silently trap them. Perhaps instead you should structure code like this:

```
try:
    maximum_size = configured_volume.pop('maximum_size')
except KeyError:
    maximum_size = None
else:
    # ... do remaining parsing here
```