Add management API for measures

[denicaM]

When creating a new measure the dashboard has to add it in 3 plans: metering, pricing and rating. Additionally we have to add a price for a measure. All this means that a lot of logic about the plan structure lives in the dashboard. Not only that but this also increases the time span during which we have inconsistency between the plans since dashboard has to perform several calls to provisioning plugin.

If we expose an API for management of measures this means we:

• encapsulate the logic for plans management in the provisioning plugin
• do all modifications with a single call
• can add pricing tag to each metric

* The issue originally was reported in the deprecated cf-abacus-broker repo: cloudfoundry-attic/cf-abacus-broker#47

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this:

https://www.pivotaltracker.com/story/show/155619708

The labels on this github issue will be updated when the story is started.