Make Client-ID a required header

It was decided to make Client-ID a required header. This patch adds a
new `before hook` that enforces this new rule.

Partially-Implements blueprint: api-v1.1-header-changes

Change-Id: I7b20d56812a0dff4f8ed3e6da75b62ef7c0f307e
Merge "Make Client-ID a required header"