Add bash completion for kubernetes orchestrator

[albers]

Depends on #1251 edit: #1251 was merged.

This

• adds completions for kubernetes-specific options if kubernetes orchestrator is configured
• hides completions for swarm-specific options unless swarm orchestrator is configured

The configured orchestrator is determined from the output of docker stack deploy|ls --help.
As the required help output is broken in 18.06.0 (see #1243) this work depends on #1251.

To test before #1251 is merged, just use a binary built at #1251.
I recommend using export DOCKER_STACK_ORCHESTRATOR=kubernetes|swarm|all, then invoking the completions of docker stack (deploy|ls|ps|rm|services)? -- and docker version --.

Thanks @silvin-lubecki for fixing the help output so fast.

[codecov-io]

Codecov Report

Merging #1254 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1254   +/-   ##
=======================================
  Coverage   54.27%   54.27%           
=======================================
  Files         268      268           
  Lines       17848    17848           
=======================================
  Hits         9687     9687           
  Misses       7551     7551           
  Partials      610      610

[albers]

I removed the selective completion here although it is correct behavior.

The important part is that --bundle-file is not completed if it is not appropriate, which is already taken care for.
If someone insists on manually typing this option, he will now get it completed although it does not exist in this context.
This is an error, but given the growing complexity of hiding options based on operating system, client/server experimental state and orchestrator replicating the hiding logic in the individual option's completions will bloat the code without much value.

[silvin-lubecki]

SGTM, thank you @albers !

[albers]

rebased in order to pull in #1251.

[thaJeztah]

LGTM 🤗