Add driver parameter for PLUMgrid configuration file
Add driver parameter for PLUMgrid configuration file

PLUMgrid plugin for Neutron supports to load dynamically a back-end driver.
This parameter has been added in the plugin configuration file.

Change-Id: I1698a8fa558841aeb94476bc8c7f103b21f2a2d3
Closes-bug: #1315466

openstack-dev/devstack,herrsechs/devstack,virtualopensystems/devstack,NaokiMiyata/devstack,promptworks/devstack,TwinkleChawla/devstack,thomasem/devstack,eharney/devstack,BSJAIN92/OpenStack,flavio-fernandes/devstack,bq-xiao/devstack,j4/devstack,mc2014/devstack,nawawi/openstack,liuquansheng47/devstack,avvocatodemarchis/devstack,liuquansheng47/devstack,pombredanne/devstack,NaokiMiyata/devstack,ozamiatin/devstack,mc2014/devstack,eharney/devstack,bigswitch/devstack,parul24/devstack,kplus/devstack-moonshot,andrewrothstein/devstack,sequenceiq/devstack,ozamiatin/devstack,bswartz/devstack,pacinete/devstack,CloudServer/devstack,bljgaurav/openstack-test,rushiagr/devstack,bigswitch/devstack,pczerkas/devstack,bljgaurav/openstack-test,nawawi/openstack,richliu/devstack,varunarya10/devstack,promptworks/devstack,neerja28/Devstack_GlusterFS,vishnugonela/devstack,dirkmueller/devstack,pombredanne/devstack,dtroyer/devstack,noironetworks/devstack,tianweizhang/devstack,Mahito/devstack,varunarya10/devstack,upadhyay-prashant/devstack,parul24/devstack,dtroyer/devstack,HPCHub/devstack,wenhuizhang/devstack,pczerkas/devstack,avvocatodemarchis/devstack,prithivm/branch01,BSJAIN92/OpenStack,innodee/openstackk,bswartz/devstack,nuxeh/devstack,LoHChina/devstack,neerja28/Devstack_GlusterFS,bq-xiao/devstack,j4/devstack,group-policy/devstack,sc68cal/devstack,pacinete/devstack,sequenceiq/devstack,mssumanth/devstack,olivierlemasle/devstack,rushiagr/devstack,group-policy/devstack,vishnugonela/devstack,atulpatil301/devstack,richliu/devstack,prithivm/branch01,costingalan/devstack,samgoon/devstack,olivierlemasle/devstack,upadhyay-prashant/devstack,atulpatil301/devstack,TwinkleChawla/devstack,Mahito/devstack,kplus/devstack-moonshot,openstack-dev/devstack,nuxeh/devstack,affo/devstack,tianweizhang/devstack,mmasaki/devstack,vsham20/devstack,wenhuizhang/devstack,sc68cal/devstack,samgoon/devstack,HPCHub/devstack,srics/devstack,costingalan/devstack,affo/devstack,thomasem/devstack,LoHChina/devstack,mssumanth/devstack,CloudServer/devstack,noironetworks/devstack,mmasaki/devstack,flavio-fernandes/devstack,dirkmueller/devstack,herrsechs/devstack,vsham20/devstack,andrewrothstein/devstack,eshnil2000/devstack