Wire in ConfKeyManager._generate_hex_key!

Rename _get_hex_key to _generate_hex_key so that this
key manager works with the super class it extends.

In making this change we also need to initialize the hex key
in the constructor before initializing the super class. Not
doing so causes test failures.

Fixes bug: 1224602

Change-Id: I9a6bd9bf96bf74a08e607af811ebbe9622b994f3
Merge "Wire in ConfKeyManager._generate_hex_key!"