Add option to manage environment variables

[drazendee]

Allows users to:

• List environment variables of a project
• Delete an env var by name
• Create a new env var (optional: set as secret)

[codecov]

Codecov Report

Merging #251 (46f2b38) into master (cdbcbed) will decrease coverage by 0.54%.
The diff coverage is 51.66%.

@@            Coverage Diff             @@
##           master     #251      +/-   ##
==========================================
- Coverage   86.64%   86.10%   -0.55%     
==========================================
  Files         125      129       +4     
  Lines        4186     4246      +60     
  Branches      684      688       +4     
==========================================
+ Hits         3627     3656      +29     
- Misses        384      414      +30     
- Partials      175      176       +1     

Impacted Files	Coverage Δ
...cli/commands/project/environment_variables/list.py	37.50% <37.50%> (ø)
...i/commands/project/environment_variables/delete.py	52.94% <52.94%> (ø)
...i/commands/project/environment_variables/create.py	60.00% <60.00%> (ø)
...commands/project/environment_variables/__init__.py	100.00% <100.00%> (ø)
valohai_cli/commands/parcel.py	65.71% <0.00%> (-1.91%)	⬇️

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