Limited use trusts

Trusts now have a "remaining_uses" field that tracks how many times
a trust can still issue a token. It is decremented by 1 each time a
trust related authentication occurs (call to /auth/tokens), until it
reaches 0 and no token can be issued through this trust anymore. If
set to null (default value), trusts can be used indefinitely to
authenticate.
This is the client side of the implementation.

Closes-Bug: #1250617
Implements: bp trusts-chained-delegation

Change-Id: Ib035a9772b7f035c3a9af102e8e15a860a96a96d
Merge "Limited use trusts"