Use workspace instead of env for tf >=0.10

[lkysow]

• Use workspace instead of env for TF versions >= 0.10 since this was deprecated in 0.10 and is not allowed in 0.11
• Use the word Workspace everywhere in the codebase to avoid confusion

Fixes #197

[codecov-io]

Codecov Report

Merging #219 into master will increase coverage by 0.61%.
The diff coverage is 72.47%.

@@            Coverage Diff             @@
##           master     #219      +/-   ##
==========================================
+ Coverage    63.5%   64.11%   +0.61%     
==========================================
  Files          39       38       -1     
  Lines        1759     1750       -9     
==========================================
+ Hits         1117     1122       +5     
+ Misses        592      578      -14     
  Partials       50       50

Impacted Files	Coverage Δ
server/events/markdown_renderer.go	91.3% <ø> (ø)	⬆️
server/events/apply_executor.go	0% <0%> (ø)	⬆️
server/events/atlantis_workspace.go	0% <0%> (ø)
server/events/run/run.go	61.11% <100%> (ø)	⬆️
server/events/atlantis_workspace_locker.go	100% <100%> (ø)
server/server.go	72.3% <100%> (ø)	⬆️
server/events/pull_closed_executor.go	94.59% <100%> (ø)	⬆️
server/events/project_pre_execute.go	96.07% <100%> (ø)	⬆️
server/events/locking/locking.go	100% <100%> (ø)	⬆️
server/events/command_handler.go	89.65% <100%> (ø)	⬆️
... and 5 more

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