Finish Usage Price Override feature

[pierre]

See:

• Issue 614 : Usage price override #635
• Usage price override #638
• Unit price override #665
• Adding some DAO level tests and fixing few small issues for unit price override #676

[shaun-forgie]

We have just uncovered a bug if a plan has more than one usage type you have to submit all usage type records as part of the API request to override the price. If you have 10 or more usage types for a plan this can be a problem. See #1956 for more info.

[shaun-forgie]

I have also created a ticket for capacity usage price overrides here #2010