This adds a "validation_utils" module to cloudkitty, which aims at
centralizing common voluptuous helpers.
Work items:
* Create the validation_utils module, and move the "get_string_type" helper
function to it. API-specific helpers have not been moved.
* Update the module of the "get_string_type" helper function in the
documentation.
* Add two validators to the "validation_utils" module, allowing to check
the types of a dict's values and keys.
Change-Id: I938f78b9987fcc4752b84cff8881ceecb5e20caf
Story: 2005890
Task: 35657
e05c00e117