Add --tf-download-url flag

[lkysow]

Add new --tf-download-url flag that configures where Terraform is downloaded from. Defaults to https://releases.hashicorp.com. This can be used in environments where calls to https://releases.hashicorp.com are firewalled.

Notes:

• Fixes Allow configuring custom releasesURL #614
• Carries @cullenmcdermott's work from Add --tf-download-url configuration option #787
• Closes Add --tf-download-url configuration option #787

[codecov]

Codecov Report

Merging #903 into master will increase coverage by 0.02%.
The diff coverage is 78.57%.

@@            Coverage Diff             @@
##           master     #903      +/-   ##
==========================================
+ Coverage   71.83%   71.86%   +0.02%     
==========================================
  Files          65       65              
  Lines        5213     5218       +5     
==========================================
+ Hits         3745     3750       +5     
  Misses       1183     1183              
  Partials      285      285

Impacted Files	Coverage Δ
server/user_config.go	100% <ø> (ø)	⬆️
server/server.go	63.69% <100%> (+0.11%)	⬆️
cmd/server.go	78.97% <100%> (+0.19%)	⬆️
server/events/terraform/terraform_client.go	78.15% <72.72%> (+0.21%)	⬆️

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