Autodownload tf versions. Add --default-tf-version.

[lkysow]

This changeset has two features:

1. We now automatically download the version of terraform specified in
   atlantis.yaml configs if we don't already have that version available
   locally.

2. Add a new --default-tf-version flag that allows users to set a
   default version of Terraform that we will also download if it's not on
   disk.

These mean that users don't need to build custom Docker images to just
add terraform versions. It also means that upgrading the version of
terraform that is packaged with the Atlantis Docker image won't cause
issues for existing users because as long as they're running with
--default-tf-version, Atlantis will always use that version.

Fixes #120

[codecov]

Codecov Report

Merging #538 into master will increase coverage by 0.95%.
The diff coverage is 77.08%.

@@            Coverage Diff             @@
##           master     #538      +/-   ##
==========================================
+ Coverage   72.18%   73.13%   +0.95%     
==========================================
  Files          62       57       -5     
  Lines        4516     4259     -257     
==========================================
- Hits         3260     3115     -145     
+ Misses       1012      915      -97     
+ Partials      244      229      -15

Impacted Files	Coverage Δ
server/user_config.go	100% <ø> (ø)	⬆️
cmd/server.go	79.08% <100%> (+0.13%)	⬆️
server/server.go	67.65% <100%> (ø)	⬆️
server/events/terraform/terraform_client.go	79.89% <75.55%> (+1.02%)	⬆️
server/logging/simple_logger.go	0% <0%> (-88.89%)	⬇️
server/recovery/recovery.go	0% <0%> (-81.82%)	⬇️
server/events/vcs/proxy.go	0% <0%> (-28.58%)	⬇️
server/events/command_runner.go	52.77% <0%> (-14.36%)	⬇️
server/events/models/models.go	72.8% <0%> (-12.8%)	⬇️
server/events/working_dir.go	74.69% <0%> (-9.64%)	⬇️
... and 8 more

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