Refactor plugin initialization and enablement

Refactor plugin initialization and enablement

Move most plugin enablement and initialization code in plugin.py to
reduce code duplication and simplify and unifify plugin enablement
for all base plugin types (login, info, providers).

This patch breaks backwards compatibility as it changes how the list
of enabled plugins is stored in the database tables.

Signed-off-by: Simo Sorce <simo@redhat.com>
Reviewed-by: Patrick Uiterwijk <puiterwijk@redhat.com>