Remove kwargs from trust_api.create_trust

This change allows notifications to be sent using the existing
implementation and the resulting trust ID is stored in args[1], instead
of kwargs. Using args this way maintains consistency with
trust_api.delete_trust().

Change-Id: I8169b9ea6fabcd8aff864c3bf900ceb1b32676f3
Closes-Bug: #1269157
Implement notifications for trusts

Use the existing notifications wrapper to send notifications when a
trust is created or deleted. There is no need to send a notification on
a trust update since trusts are immutable. This change adds
trusts to the event_notifications.rst documentation.

Change-Id: I777ad4594ce9b93c36af675ac677cfa61524a3e7
blueprint: trust-notifications
Merge "Remove kwargs from trust_api.create_trust"
Merge "Implement notifications for trusts"