Plugins should standardize their development sqlite locations

[EliAndrewC]

Moved here from the original (now private) issue tracker.

Sideboard has a db directory with a README file which explains "Directory for plugins' sqlite files. Sideboard exposes this directory as a config option called 'sqlite_dir'."

However, our default development-defaults.ini file for plugins actually puts the file at /tmp/PLUGIN_NAME.db which is also fine, but doesn't agree with the above.

Also, some of our unit tests put the test sqlite file in PLUGIN_ROOT/data which is also fine, but doesn't agree with either of the above.

We should make all three of these things the same. I don't really care which, so long as the current production config remains unchanged (/opt/sideboard/db).