Manages salt service during watchmaker install

[lorengordon]

• The salt-minion service is stopped before managing formulas
• If the service is stopped (or not-yet-installed) before installing salt, the service remains stopped after watchmaker completes
• If the service is running prior to the install (someone has intentionally started the service, and re-run watchmaker), then it remains running after watchmaker completes

Fixes #234
Fixes #72

[codecov]

Codecov Report

Merging #237 into develop will decrease coverage by 0.66%.
The diff coverage is 14.28%.

@@             Coverage Diff             @@
##           develop     #237      +/-   ##
===========================================
- Coverage    19.12%   18.45%   -0.67%     
===========================================
  Files           14       14              
  Lines          774      818      +44     
  Branches        79       89      +10     
===========================================
+ Hits           148      151       +3     
- Misses         625      666      +41     
  Partials         1        1

Impacted Files	Coverage Δ
src/watchmaker/workers/salt.py	14% <12.5%> (-0.53%)	⬇️
src/watchmaker/managers/base.py	18.3% <17.64%> (-1.29%)	⬇️

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