User experience improvement

CK User experience improvement
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The purpose of this blueprint is to improve the user experience by providing
more insight of hashmap functionalities.

Current situtation:

When creating rating rules, the user may need the following information:

- which services are activated ?
- which metadata can be used in field mapping ?
- According which period of time a flat entry is applied ?

Furthermore, rating preview in instance creation wizard is only a price without any time period information.

Today, the only way to get those information is to look at CK code or the
CK config file (when having access to the server it runs on).
Improvements have been made on the documentation during the last cycle
but it may not be sufficient (and by experienced often obsolete).

Solution:

One solution is to provides all necessary operational information through
a dedicacted API, CLI command and horizon wizard.

Work item insights:

* add api entrypoints for listing cloudkitty current configuration values (available
    services, collection period, etc)
* add metadata property to collector or transformer classes for a given service
    (each implementation provides its own metadata description)
* add api entrypoints for listing collector metadata field names for a given
    service (ie transformed identifiers, units, etc)
* add pythonclient models and CLI command
* modify horizon to provides either a rule creation wizard or drop-down list in current views