The change from I7b384c2f78aa0aa622d23b8b4ee317dd99f396e0 in the
neutron-lbaas-dashboard brought back the ``enable_lb`` option. This
patch adds the option back into the Horizon configuration. The option
is only needed when the next-gen LBaaSv2 panels are activated.
Closes-bug: 1593074
Change-Id: I166512fc77e48c123116dc6dd95262b2e1d5f6cf