Create JSON request for API

This commit adds a method that forms a JSON request content string
in the correct format containing a list of test IDs that have
passed Tempest. Consequently, a method was created that uses
the Keystone client to get the Keystone ID which will be used
as the cpid.

Change-Id: I445415af8f0ecdad68b3ad2860cbe8da8a0dfe0a