Add ps dependency to plugin-dev Docker image for pm2

[benoitvidis]

What does this PR do ?

pm2 has an optional dependency on ps tool.
When pm2 tries to restart a process, for instance if a watched file changes, we get an exception in the log trace (but still manages to restart the service):

{ Error: spawn ps ENOENT
    at _errnoException (util.js:992:11)
    at Process.ChildProcess._handle.onexit (internal/child_process.js:190:19)
    at onErrorNT (internal/child_process.js:372:16)
    at _combinedTickCallback (internal/process/next_tick.js:138:11)
    at process._tickDomainCallback (internal/process/next_tick.js:218:9)
  code: 'ENOENT',
  errno: 'ENOENT',
  syscall: 'spawn ps',
  path: 'ps',
  spawnargs: [ '-o', 'pid', '--no-headers', '--ppid', 92 ] } 

How should this be manually tested?

1. Build the image locally

docker build -t kuzzleio/plugin-dev:latest --target plugin-dev .

2. Test the plugin boiler plate does not throw errors anymore on file change detection.

[codecov]

Codecov Report

Merging #1317 into 1-dev will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           1-dev   #1317   +/-   ##
=====================================
  Coverage   93.8%   93.8%           
=====================================
  Files         98      98           
  Lines       6991    6991           
=====================================
  Hits        6558    6558           
  Misses       433     433

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[codecov]

Codecov Report

Merging #1317 into 1-dev will decrease coverage by 0.25%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           1-dev    #1317      +/-   ##
=========================================
- Coverage   93.8%   93.55%   -0.26%     
=========================================
  Files         98       98              
  Lines       6991     7090      +99     
=========================================
+ Hits        6558     6633      +75     
- Misses       433      457      +24

Impacted Files	Coverage Δ
lib/api/core/notifier.js	74.13% <0%> (+1.2%)	⬆️

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[scottinet]

@benoitvidis > I took the liberty to remove the "enhancement" tag for this PR, so that it would be listed in the Others part of our changelogs, since I think it's not really an enhancement. Feel free to put a tag again if you disagree.