Fix apply all with project name bug

[lkysow]

Previously, if you were using project names and you had two projects
with the same directory and workspace but different workflows, running
"atlantis apply" would fail. This was because we weren't figuring out
what the project name was for each pending plan.

This change fixes this by extracting the project name from the planfile.
We also handle /'s in the project name by substituting them with '::'.
This allows us to convert them back to /'s when we're figuring out the
project names from the filenames. It's safe to do this because : isn't
an allowed character for project names.

We also remove a final substitution of invalid filename characters
because we rely on the validation of the project names instead which
happens when validating the atlantis.yaml file.

Fixes #365

[codecov]

Codecov Report

Merging #565 into release-0.7 will decrease coverage by 0.02%.
The diff coverage is 72.72%.

@@               Coverage Diff               @@
##           release-0.7     #565      +/-   ##
===============================================
- Coverage         72.2%   72.17%   -0.03%     
===============================================
  Files               61       61              
  Lines             4587     4597      +10     
===============================================
+ Hits              3312     3318       +6     
- Misses            1033     1035       +2     
- Partials           242      244       +2

Impacted Files	Coverage Δ
server/events/yaml/valid/repo_cfg.go	19.04% <0%> (ø)	⬆️
server/events/project_command_builder.go	82.17% <100%> (ø)	⬆️
server/events/pending_plan_finder.go	78.04% <71.42%> (-3.54%)	⬇️
server/events/runtime/runtime.go	78.94% <83.33%> (-4.39%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b1ffb66...609bf84. Read the comment docs.