Use call_command to invoke loaddata.

When triggering loaddata from rb-site, we were using an old method of invoking
the command which wouldn't set up default values for options. This change
updates us to use `call_command()` instead of `handle()`

Testing done:
Smoke tested rb-site install.

Fixes bug 3126.

Reviewed at https://reviews.reviewboard.org/r/5356/