Build the default database path from STORAGE_DIR

Currently, the default database path is hardcoded to /opt.  In a
development installation, graphite-web may be installed anywhere
and will fail to start following the development installation
instructions.  Using STORAGE_DIR aligns this value with the
comments earlier in the file.

- Changed definition of DATABASES

- Moved DATABASES definition to after STORAGE_DIR definition.
Don't overwrite local_settings.py defined DATABASES

As per the discussion in PR #937.
Merge pull request #937 from andrewbaxter/settings_default_db_path

Build the default database path from STORAGE_DIR