Catch EtcdKeyNotFound in addition to EtcdCompareFailed.
Catch EtcdKeyNotFound in addition to EtcdCompareFailed.

Back-application of networking-calico
7b4b9a7721fa953a59da06780c1abf10a74ec995.

Fixes #914.

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