Add loan management CLI

Adds "bkr loan-grant" and "bkr loan-return" commands.

Also adds a private RESTful API for loan management. This
will be made public once loans exist as a separate part of
the data model, with recorded start and finish dates.

Bug: 734212
Change-Id: I150083a00dd6ea510ebe8eb0e2516aa44121e7cb