v2_cart_model: allow archiving of destroyed gears

Adds cart model logic and two node config options to allow admins to
enable archiving of gears as they are destroyed, and to specify the
directory for archiving the gears.

Note that this doesn't archive the user env vars
(`GEAR_DIR/.env/user_vars`) as they are un-set before
`V2CartridgeModel#destroy` gets called. The user env vars can be
determined from looking at the node logs.

Enterprise bug: 1092045
https://bugzilla.redhat.com/show_bug.cgi?id=1092045
Conventionally, we always specify a default value to `get` and `get_bool`.  For `ARCHIVE_DESTROYED_GEARS_DIR` in particular, that could make the difference between getting a `TypeError` exception or not (although I could only see that happening if the admin explicitly set `ARCHIVE_DESTROYED_GEARS=true` and did not set `ARCHIVE_DESTROYED_GEARS_DIR`).