Persistent / replicated metadata, util init fixes, and documentation

[chungers]

This PR

• Addresses the issue (Infrakit template command different from utils init #729)
• Adds the --metadata and --persist flags to set and persist metadata.
• util init will parse the groups spec and starts the plugin automatically. If --persist option
  is specified, it will start up the manager, vars, and group plugins as pre-conditions for processing the input spec.
• Added documentation -- see docs/cmd/infrakit/util/init/README.md.

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

[codecov]

Codecov Report

Merging #730 into master will decrease coverage by 0.07%.
The diff coverage is 46.42%.

@@            Coverage Diff             @@
##           master     #730      +/-   ##
==========================================
- Coverage   47.73%   47.66%   -0.08%     
==========================================
  Files          89       89              
  Lines        9879     9895      +16     
==========================================
  Hits         4716     4716              
- Misses       4807     4822      +15     
- Partials      356      357       +1

Impacted Files	Coverage Δ
pkg/types/interface_spec.go	0% <0%> (ø)	⬆️
pkg/core/addressable.go	90% <100%> (+6.17%)	⬆️
pkg/template/template.go	64.4% <100%> (+0.28%)	⬆️
pkg/rpc/mux/server.go	43.33% <0%> (-4.17%)	⬇️
pkg/types/spec.go	51.82% <0%> (-2.19%)	⬇️

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