The majority of the start.sh code is identical. This removes that
duplicate code while still maintaining the ability to call code in a
specific container.

The start.sh is moved into /usr/local/bin/kolla_start in the container

The extend_start.sh script is called by the kolla_start script at the
location /usr/local/bin/kolla_extend_start . It always exists because
we create a noop kolla_extend_start in the base directory. We override
it with extend_start.sh in a specific image should we need to.

Of note, the neutron-agents container is exempt from this new
structure due to it being a fat container.

Additionally, we fix the inconsistent permissions throughout. 644 for
repo files and the scripts are set to 755 via a Docker RUN command to
ensure someones local perm change won't break upstream containers.

Change-Id: I7da8d19965463ad30ee522a71183e3f092e0d6ad
Closes-Bug: #1501295