Move python-etcd dependence to calico-common.
Move python-etcd dependence to calico-common.

Prevent upgrade of the actual Calico code unless python-etcd
is also updated.

Fixes #812.

neiljerram/felix,alexhersh/calico,neiljerram/felix,Metaswitch/calico,TrimBiggs/calico,alexhersh/calico,neiljerram/felix,TrimBiggs/calico,Metaswitch/calico,neiljerram/felix,matthewdupre/felix,matthewdupre/felix