Add a new registration interface to service_clients.
Add a new optional method to the plugin interface, that exposes
the plugin service client registration details.
Tests in plugins can initialise service_clients with parmaters
common to their service clients and other ones they may need.
Parameters specific to their service clients are passed via the
registration interface, and can be overwritten at any time
by passing extra parameters at client init time.
Partially-implements: bp client-manager-refactor
Change-Id: I2d99aaa317b0d21c0968dd25b21c4ba9088136fb
Andrea Frittoli (andreaf)