Make settings_example.py Usable for Local Development

Ideally developers should require as little setup as possible
to get starting working on relengapi. This change will default
the settings to create a sqlite database in the current directory.
I'd love a comment here, above this
# Edit the next line to specify a proper path/db name

And then using 'relengapi.db' instead of 'my.db'

For the path, I'd rather you do
`os.path.join( os.path.dirname(os.path.abspath(__file__)), 'relengapi.db')` 

This gets us "wherever settings.py is, create a database right next to it" which is more discoverable that "curdir" since you could be at ~ or /root or ~/my-relengapi-repo/foo/bar/baz/something/ when you run once, and elsewhere the next time, suddenly losing your db.  It also doesn't follow symlink (e.g. if you do ln -s ... ) basically enforcing 'wherever RELENGAPI_SETTINGS is defined to point'

I'd normally want this to be the in-memory we have for "no settings file" but since the existing value here doesn't work at all, I'm ok with it given these changes.