Decorate methods in ExtensionDescriptor with abstractmethod
Decorate methods in ExtensionDescriptor with abstractmethod

Change-Id: I1cbde0c622d5595664f8547284c2c765de2f4c70
Closes-Bug:1516283

noironetworks/neutron,mahak/neutron,cloudbase/neutron,MaximNevrov/neutron,igor-toga/local-snat,sebrandon1/neutron,sebrandon1/neutron,dims/neutron,bigswitch/neutron,wolverineav/neutron,klmitch/neutron,openstack/neutron,igor-toga/local-snat,eayunstack/neutron,noironetworks/neutron,mahak/neutron,huntxu/neutron,bigswitch/neutron,mahak/neutron,huntxu/neutron,eayunstack/neutron,klmitch/neutron,openstack/neutron,MaximNevrov/neutron,cloudbase/neutron,wolverineav/neutron,openstack/neutron,dims/neutron