Support for ssh

[chungers]

This PR adds support for executing commands over ssh on remote hosts.

• While we strongly prefer immutable infrastructure, ssh is sometimes required to manage the configurations of provisioned nodes (ansible style).
• Support for basic password / private key and ssh agent auths
• Integration with playbook as a backend choice
• Backends can add additional flags that are applied for all playbooks using that backend (e.g. ssh remote host, user, password, etc.)
• Support for multiple SSH remotes and execution of scripts in parallel.

Documentation: See pkg/cli/backend/ssh/README.md on how to create playbooks using SSH as backend. Examples are also provided (see test.ikt).

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

[thebsdbox]

awesome!

[codecov[bot]]

Codecov Report

Merging #817 into master will decrease coverage by 1.68%. The diff coverage is 3.26%.

@@            Coverage Diff             @@
##           master     #817      +/-   ##
==========================================
- Coverage   48.87%   47.19%   -1.69%     
==========================================
  Files          84       89       +5     
  Lines        7624     7904     +280     
==========================================
+ Hits         3726     3730       +4     
- Misses       3542     3812     +270     
- Partials      356      362       +6

Impacted Files	Coverage Δ
pkg/util/ssh/tunnel.go	4.44% <ø> (ø)
pkg/cli/playbook/modules.go	62.35% <0%> (ø)	:arrow_up:
pkg/util/ssh/auth.go	0% <0%> (ø)
pkg/util/ssh/exec.go	0% <0%> (ø)
pkg/util/ssh/conn.go	0% <0%> (ø)
pkg/cli/backend/ssh/script.go	1.23% <1.23%> (ø)
pkg/util/exec/exec.go	29.37% <21.21%> (-2.65%)	:arrow_down:
pkg/types/spec.go	58.4% <0%> (-1.77%)	:arrow_down:
... and 3 more

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