Add extra config yaml files for big switch agents.

Add extra config yaml files for big switch agents.

This change adds extra config yaml files for big switch agent
and big switch lldp.

This change is mainly for compute nodes. The changes related
to controller nodes are landed at e78e1c8d9b5a7ebf327987b22091bff3ed42d1c1

This change also removes the neutron_enable_bigswitch_ml2 flag. Instead,
User needs to specify NeutronMechanismDrivers: bsn_ml2 in environment file.

Previous discussion about this change can be found at an abandoned
review request https://review.openstack.org/#/c/271940/

Depends-On: Iefcfe698691234490504b6747ced7bb9147118de

Change-Id: I81341a4b123dc4a8312a9a00f4b663c7cca63d7c