Support for determining leader location

[chungers]

This PR is necessary for upcoming API work because

• It adds a registration mechanism for leader's location (as a URL). The leader's location, which is how the client or other followers can forward http traffic to the leader, can now be stored in the backend that is used for leadership detection in the first place.
  • os: a file now stores the location of the leader node
  • swarm: a new Swarm annotation is added
  • etcd: a value at a given key infrakit/leader/location now stores the url to contacting the leader.
• The poller, on determining that this current node is the leader, writes its location to the backend, if one is assigned. In the case of single node infrakit, there's no change.
• A Store interface is introduced to let an application read the location of the leader. This is not the RPC SPI, but rather a golang interface that interested applications (e.g. the Mux) will make use.
• The Mux has been adapted to support running in the three environments listed above. The reason it's the Mux that supports this and not the Manager itself, is because the Mux provides a network endpoint that is reachable and therefore is the actual component in infrakit that really depends on this feature. If the Mux is never run, then existing leader detection mechanism is sufficient.
• The Mux CLI has been updated to plumb through the Store and leadership detections. This will allow the Mux running on a non-leader (follower) to be able to determine the location of the leader and forward client requests to the leader, so that when leadership changes, the client only has to contact a single, stable IP address.

Signed-off-by: David Chung david.chung@docker.com

[codecov]

Codecov Report

Merging #607 into master will decrease coverage by 0.46%.
The diff coverage is 23.8%.

@@            Coverage Diff             @@
##           master     #607      +/-   ##
==========================================
- Coverage    47.3%   46.84%   -0.47%     
==========================================
  Files          91       87       -4     
  Lines        6204     6197       -7     
==========================================
- Hits         2935     2903      -32     
- Misses       2969     3000      +31     
+ Partials      300      294       -6

Impacted Files	Coverage Δ
pkg/leader/etcd/v3/etcd.go	0% <0%> (ø)	⬆️
pkg/rpc/mux/server.go	61.66% <71.42%> (+3.13%)	⬆️
pkg/provider/google/plugin/gcloud/tags.go	82.14% <0%> (+3.57%)	⬆️

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