Install and run servo-gecko-try on its own minion

[edunham]

Roughly the same setup as intermittent-tracker, with the notable difference
that we manage ssh keys for the user and they live in the pillar.

I fully expect the tests to break at first and will ping someone when I think it's properly ready for review -- feel free to ignore this PR for now.

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This change is 

[aneeshusa]

I haven't looked at the rest of this, but I would like this to use a separate user (servo-gecko-try) instead of the servo user, as part of #421.

[edunham]

User stuff fixed as requested. I would like to land this soon to get it deployed and make life easier for the Gecko devs if possible.

[aneeshusa]

I'll check this out in Vagrant and likely have more feedback on the PR. Also, we'll want to add requisites to a bunch of the states to ensure correct ordering.

[aneeshusa]

This is no longer needed since we make a separate user account.

[aneeshusa]

Update both to servo-gecko-try.

[aneeshusa]

/home/servo-gecko-try

[aneeshusa]

/home/servo-gecko-try

[aneeshusa]

Update these paths to be under /home/servo-gecko-try.

[aneeshusa]

Move this under the servo-gecko-try ID. The name parameter can then be omitted.
TODO for me: I need to double check if this will also create the group automatically.

[aneeshusa]

Use pkgs instead of names, per the style guide. This can also get moved under servo-gecko-try.

[aneeshusa]

Also, add git here and remove it from top.sls.

[aneeshusa]

Can we get this published to PyPi?

[aneeshusa]

Can we update the identity to match bors-servo?
Also update the key location.

[edunham]

VCS sync is no more, so try is no longer needed.

[eliasp]

Typo: tlpdir → tpldir