Infer Buildbot workers from environment

[aneeshusa]

Use a dictionary for the lookup.
Make the Environment class (mostly) immutable and hashable to allow
using environment as dictionary keys.

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This change is 

[aneeshusa]

r? @metajack per #721 (review)

[jdm]

This looks fine to me.

[jdm]

However:

[ FAIL ] Buildbot master config check failed:

         Configuration Errors:

           configuration file '/home/servo/buildbot/master/master.cfg' does not exist

[ FAIL ] Buildbot master config lint check failed:

         ./buildbot/master/files/config/environments.py:25:12: E111 indentation is not a multiple of four

         ./buildbot/master/files/config/environments.py:26:12: E111 indentation is not a multiple of four

[jdm]

Renaming the configuration file will require further changes elsewhere.

[aneeshusa]

Oops, renaming the file was meant to be a local-only change for debugging, I had inadvertently committed that before and have removed it now. Yay for tests that catch silly mistakes!

[bors-servo]

☔ The latest upstream changes (presumably #850) made this pull request unmergeable. Please resolve the merge conflicts.