Plugin management utility - plugin stop

[chungers]

This PR tries to address a common complaint about running Infrakit: starting and stopping of plugins. Plugin servers now write a <name>.pid file in the same plugin discovery directory. For non-containerized cases, the pid is used by a new command line verb infrakit plugin stop to send SIGTERM to the plugin processes for force clean shutdown. On shutting down the PID file is removed cleanly as is with the socket file.

• CLI / mains now write a PID file (see pkg/cli/serverutil.go) and cleanup on shutdown.
• Change discovery to be less noisy when seeing a non-socket file (which can be a pid file).
• Added a new verb stop under infrakit plugin. This simply reads the pid files and send SIGTERM to the process: in essence it does kill -TERM $(cat /plugins/plugin.pid).
• Incorporated this command in the e2e script - scripts/e2e-test.sh.

This is useful for stopping the plugins when the starter infrakit plugin start exits (no --wait option) and the plugins become orphaned (adopted by init). Another applicable scenario is when the config json (e.g. scripts/e2e-test-plugins.json) used by the start utility has specified the plugins be running in a separate process group -- which also will leave the plugins running after the starter exits.

Workflow:

$ ~/projects/src/github.com/docker/infrakit/build/infrakit plugin start --os --config-url file://$PWD/scripts/e2e-test-plugins.json group-default
Starting up group-default
# .... lots of logs
$ ~/projects/src/github.com/docker/infrakit/build/infrakit plugin ls
NAME                	LISTEN
group-stateless     	/Users/me/.infrakit/plugins/group-stateless
~/projects/src/github.com/docker/infrakit$ build/infrakit plugin stop group-stateless
# ... logs

[codecov-io]

Current coverage is 65.14% (diff: 25.00%)

Merging #367 into master will decrease coverage by 0.60%

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