Add use_slave DB api support
Add use_slave DB api support

The example configuration refers to slave_connection[1] but slaves aren't used
in the codebase. This change will enable plugin authors to use
slave_connection by passing use_slave=True to the SQLAlchemy session[2][3].

[1] http://docs.openstack.org/juno/config-reference/content/section_neutron.conf.html
[2] http://docs.openstack.org/developer/oslo.db/api/sqlalchemy/session.html#oslo_db.sqlalchemy.session.EngineFacade.get_session
[3] https://wiki.openstack.org/wiki/Slave_usage

Change-Id: I6f46c11fad5c58577654a4011cf82d19f6d3e1e3

eayunstack/neutron,silenci/neutron,openstack/neutron,glove747/liberty-neutron,NeCTAR-RC/neutron,igor-toga/local-snat,cloudbase/neutron,jumpojoy/neutron,MaximNevrov/neutron,eonpatapon/neutron,jacknjzhou/neutron,watonyweng/neutron,wenhuizhang/neutron,huntxu/neutron,chitr/neutron,skyddv/neutron,eonpatapon/neutron,jerryz1982/neutron,vivekanand1101/neutron,bigswitch/neutron,glove747/liberty-neutron,mahak/neutron,shahbazn/neutron,yanheven/neutron,MaximNevrov/neutron,swdream/neutron,dims/neutron,dims/neutron,eayunstack/neutron,suneeth51/neutron,asgard-lab/neutron,klmitch/neutron,SamYaple/neutron,sasukeh/neutron,javaos74/neutron,JianyuWang/neutron,NeCTAR-RC/neutron,igor-toga/local-snat,mandeepdhami/neutron,barnsnake351/neutron,cloudbase/neutron,wolverineav/neutron,noironetworks/neutron,chitr/neutron,mandeepdhami/neutron,JioCloud/neutron,sebrandon1/neutron,noironetworks/neutron,adelina-t/neutron,klmitch/neutron,skyddv/neutron,swdream/neutron,paninetworks/neutron,dhanunjaya/neutron,apporc/neutron,neoareslinux/neutron,wenhuizhang/neutron,mahak/neutron,bigswitch/neutron,mahak/neutron,apporc/neutron,mmnelemane/neutron,mattt416/neutron,openstack/neutron,paninetworks/neutron,neoareslinux/neutron,openstack/neutron,dhanunjaya/neutron,shahbazn/neutron,silenci/neutron,jacknjzhou/neutron,cisco-openstack/neutron,bgxavier/neutron,sasukeh/neutron,JioCloud/neutron,cisco-openstack/neutron,watonyweng/neutron,yanheven/neutron,suneeth51/neutron,sebrandon1/neutron,SamYaple/neutron,asgard-lab/neutron,barnsnake351/neutron,mmnelemane/neutron,bgxavier/neutron,adelina-t/neutron,huntxu/neutron,takeshineshiro/neutron,jumpojoy/neutron,takeshineshiro/neutron,mattt416/neutron,vivekanand1101/neutron,wolverineav/neutron,javaos74/neutron,jerryz1982/neutron,JianyuWang/neutron