Improve completion of service scale args

[albers]

Without this PR:
docker service scale <TAB> completes service names, followed by a space. This tempts the user to enter a number, producing illegal syntax, e.g. docker service scale nginx 2.

With this PR:
docker service scale <TAB> completes service names with "=" appended and no trailing space. This guides the user to the correct argument syntax, e.g. docker service scale nginx=2.

The legacy completion worked like this.

Improve CLI completion of `docker service scale`.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 11 lines in your changes missing coverage. Please review.

Project coverage is 59.08%. Comparing base (b8034c0) to head (ee275d5).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5968      +/-   ##
==========================================
- Coverage   59.09%   59.08%   -0.02%     
==========================================
  Files         354      354              
  Lines       29733    29741       +8     
==========================================
  Hits        17572    17572              
- Misses      11191    11199       +8     
  Partials      970      970              

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[albers]

@thaJeztah The test failures seem not to be related to my changes.

[thaJeztah]

LGTM, thank you!