Remove unused threads argument
Remove unused threads argument

Keystone never uses the `threads` argument to
eventlet_server.Server(). Simplify the code to remove the argument
and just use eventlet's default (the default max_size is 1024, so
there will be 24 more greenthreads in its pool).

Change-Id: Icecec03b4599f24fbdf98c36403c1abcafce471b

ilay09/keystone,nuxeh/keystone,jonnary/keystone,jumpstarter-io/keystone,rushiagr/keystone,dstanek/keystone,promptworks/keystone,rushiagr/keystone,klmitch/keystone,cernops/keystone,maestro-hybrid-cloud/keystone,dims/keystone,mahak/keystone,roopali8/keystone,ajayaa/keystone,himanshu-setia/keystone,openstack/keystone,openstack/keystone,promptworks/keystone,takeshineshiro/keystone,ilay09/keystone,maestro-hybrid-cloud/keystone,takeshineshiro/keystone,vivekdhayaal/keystone,dstanek/keystone,jamielennox/keystone,nuxeh/keystone,vivekdhayaal/keystone,vivekdhayaal/keystone,rushiagr/keystone,rajalokan/keystone,rajalokan/keystone,klmitch/keystone,rajalokan/keystone,openstack/keystone,dims/keystone,jumpstarter-io/keystone,nuxeh/keystone,promptworks/keystone,dstanek/keystone,idjaw/keystone,himanshu-setia/keystone,mahak/keystone,roopali8/keystone,ilay09/keystone,mahak/keystone,jamielennox/keystone,idjaw/keystone,ajayaa/keystone,jonnary/keystone,jumpstarter-io/keystone,cernops/keystone