Add condition to the cluster id check
Add condition to the cluster id check

Some endpoints, most notably the /stats/ ones, do not return a
x-etcd-cluster header. While this gets fixed in etcd, we need
to not fail hard in those cases.

j-mcnally/python-etcd,jplana/python-etcd,thepwagner/python-etcd,thepwagner/python-etcd,mbrukman/python-etcd,vodik/python-etcd,projectcalico/python-etcd,ocadotechnology/python-etcd,ocadotechnology/python-etcd,projectcalico/python-etcd,vodik/python-etcd,jlamillan/python-etcd,aziontech/python-etcd,kireal/python-etcd,jlamillan/python-etcd,sentinelleader/python-etcd,dmonroy/python-etcd,sentinelleader/python-etcd,mbrukman/python-etcd,kireal/python-etcd,aziontech/python-etcd,j-mcnally/python-etcd,jplana/python-etcd,dmonroy/python-etcd