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!