Add indexes for test plans
Add indexes for test plans

Add Mongo DB indexes for the test case and test group collections to
include the "plan" field.  This is especially useful when retrieving
or counting documents that have the same test plan name.

Signed-off-by: Guillaume Tucker <e7e20b7c30f19d4ca6b81319ef81bf200369d137@collabora.com>

kernelci/kernelci-backend,kernelci/kernelci-backend