Added job to remove uninstalled packs from Mongo database

[jk464]

Useful when changing the mounted pack image, that removes previously installed packs.

Normally the packs have been removed from disk, but are still registered in the Mongo database.

This runs a job as part of registering packs, to check for any packs no longer present on disk and removes them from the DB.

[jk464]

@cognifloyd if you can give me any pointers into the st2 core where we could add those new "primitives" I'd be happy to take a stab at re-implementing this functionality there (and I'd also add some logic to ignore dunder-name packs).

When I took a stab at this originally, I did look at doing it in core, but got lost trying to follow the code to locate the logic where pack loading happens at bootstrap - and therefore where I could add pack un loading