Fix bug where Atlantis hangs on terraform crash.

[lkysow]

Because of some weirdness with how terraform executes (it forks itself
and assigns the parent's stderr and stdout file descriptors to the
child) when terraform panics, Go's exec.CombinedOutput() blocks
indefinitely even though the terraform process has exited.

To handle this, we switch to using os.Pipe for the cmd's stderr/out
which for some reason allows Go to know when the process has exited.
Unfortunately, this brings up a new problem when reading the output from
that pipe. For some reason, after a terraform crash, we never get an EOF
on the pipe and so as we again get in a situation where we're blocking
indefinitely. To handle this, I've added an edge case where we look at
the log lines and if its the end of a terraform panic, we break out of
the loop.

Thanks Terraform 0.12-alpha for discovering this bug!

[codecov]

Codecov Report

Merging #421 into master will increase coverage by 0.04%.
The diff coverage is 86.66%.

@@            Coverage Diff             @@
##           master     #421      +/-   ##
==========================================
+ Coverage   70.23%   70.27%   +0.04%     
==========================================
  Files          63       63              
  Lines        3924     3943      +19     
==========================================
+ Hits         2756     2771      +15     
- Misses        972      974       +2     
- Partials      196      198       +2

Impacted Files	Coverage Δ
server/events/terraform/terraform_client.go	75.26% <86.66%> (+0.94%)	⬆️

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