Add flag for setting Terraform Enterprise hostname

[lkysow]

--tfe-hostname will allow for creating a .terraformrc file with a
different hostname than app.terraform.io

Fixes #702

[codecov]

Codecov Report

Merging #706 into master will increase coverage by 0.06%.
The diff coverage is 93.75%.

@@            Coverage Diff             @@
##           master     #706      +/-   ##
==========================================
+ Coverage    72.4%   72.47%   +0.06%     
==========================================
  Files          61       61              
  Lines        4664     4675      +11     
==========================================
+ Hits         3377     3388      +11     
  Misses       1037     1037              
  Partials      250      250

Impacted Files	Coverage Δ
server/user_config.go	100% <ø> (ø)	⬆️
server/server.go	66.66% <100%> (+0.83%)	⬆️
cmd/server.go	79.42% <100%> (+0.4%)	⬆️
server/events/terraform/terraform_client.go	79.89% <75%> (ø)	⬆️

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