Add new -var args for tf plan.

[lkysow]

Previously we added -var atlantis_repo=runatlantis/atlantis but the '/'
character is not a valid character for the AWS assume role session name.
To fix this, this commit adds -var atlantis_repo_owner=runatlantis and
-var atlantis_repo_name=atlantis (where runatlantis and atlantis would
be the values for the repo the pull request is from ).

[codecov]

Codecov Report

Merging #300 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #300      +/-   ##
==========================================
+ Coverage    70.5%   70.54%   +0.03%     
==========================================
  Files          61       61              
  Lines        3635     3639       +4     
==========================================
+ Hits         2563     2567       +4     
  Misses        893      893              
  Partials      179      179

Impacted Files	Coverage Δ
server/events/runtime/plan_step_runner.go	93.33% <100%> (+0.47%)	⬆️

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[jolexa]

I agree it makes sense to pass the ctx into the tfVars function