Set human-readable indexes for build databases

[gregharvey]

Original ticket by @pm98zz-c reads as follows:

We currently rely on index for how we backup/gather info on databases. IE, the resulting structure (as used by settings.php and similar config files) looks like:

{{ build_databases[0].name }}
{{ build_databases[0].user }}
{{ build_databases[0].host }}
...

To make it more reliable, and cope with more use cases, we should instead rely on the database "engine" and the database "base name" as keys. So, resulting structure would look like (using cewww project as an example):

{{ build_databases.mysql.cewww_dev.name }}
{{ build_databases.mysql.cewww_dev.user }}
{{ build_databases.mysql.cewww_dev.host }}
{{ build_databases.redis.cewww_dev.host }}
{{ build_databases.postgresql.cewww_dev.host }}
{{ build_databases.memcache.cewww_dev.host }}
...

Or, in an equivalent but more portable way (pseudo syntax, needs to be checked):

{{ build_databases.mysql[project_name + '' + build_type].name }}
{{ build_databases.mysql[project_name + '' + build_type].user }}
{{ build_databases.mysql[project_name + '_' + build_type].host }}
...