Replace neutron-specific LengthStrOpt with StrOpt
Replace neutron-specific LengthStrOpt with StrOpt

max_length option was added to StrOpt in oslo.config 2.7.0.
Neutron-specific LengthStrOpt is no longer required.

Change-Id: Icfecb5c8c68c942fca847e27a0c1862f18bdcdca
Closes-Bug: #1518178

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