Adding cfn-credentials file to heat-cfntools

Change-Id: Icd9f0e24700d8016169682f86d5004ec1486c463
Refactor mysql element to do less in first-boot.d

We didn't need to do most of this anyway, we just needed to configure
mysql before we installed the packages in the image. Some of the other
things were half-baked ideas which will come in later changes to add
os-refresh-config and os-config-applier support to the element.

Change-Id: Ib1b66a8b3b70fac10f3f826b1efd414c67f9bfc6
Set mysql server_id based on instance-id

Do nothing if we are booted on a server which has no instance-id set,
such as a non-cloud VM or server.

Change-Id: I6df81edbea1b2b4e4f1ef11a9f2ddaf100d98eda
Create users in mysql server based on metadata

Enable templates to specify a list of users that need passwords assigned
in Metadata. These passwords are then communicated back to the template
via an optional wait condition handle using cfn-signal.

Change-Id: Iaaf4d4a9d0d757b7d44ea39e77eed3c55ffffd88