Use DATABASE_URL only instead of database.yaml

This makes the Dockerfile set DATABASE_URL instead of mixing it with
database.yaml which apparently doesn't work.
We choose to use DATABASE_URL over database.yaml because it's much
easier to configure/customize this in docker and similiar environments.

You can create the database by:

    $ docker run -t -i -link mysqld:db \
       DATABASE_URL="mysql2://<user>:<pass>@[HOST]/<db>" \
       rake db:setup

(You can also set DATABASE_URL if you can use docker links)

Now you can run promdash like this:

    $ docker run -t -i -link mysqld:db \
       DATABASE_URL="mysql2://<user>:<pass>@[HOST]/<db>" \
       -p 3000 promdash

[HOST] gets replaced by ip:host of the linked container.