Use girder jobs for spawning Instances

[Xarthisius]

Initial refactoring of POST/DELETE /instance. POST endpoint now returns Job object that can be used to track Instance status. Please note that creating an instance is a two step task: volume creation and container creation. Separate jobs are being created for each task. Only the latter is returned by calling POST /instance but the former can be accessed via parent keyword.

After some thought, I kept the original behavior. POST /instance still returns instance. There's a new state though: Instance.LAUNCHING (see server/constants.py for full enum). The actual spawning is done via girder jobs now and they can provide additional status if necessary (when we have interface for that).

Note: It requires corresponding branch on gwvolman (issue_95)

PLEASE DO NOT MERGE UNTIL [WIP] is removed from the title of this PR

[codecov]

Codecov Report

Merging #100 into master will increase coverage by 1.54%.
The diff coverage is 30.43%.

@@            Coverage Diff             @@
##           master     #100      +/-   ##
==========================================
+ Coverage   37.42%   38.97%   +1.54%     
==========================================
  Files          18       18              
  Lines        1245     1270      +25     
==========================================
+ Hits          466      495      +29     
+ Misses        779      775       -4

Impacted Files	Coverage Δ
server/__init__.py	42.51% <100%> (+0.69%)	⬆️
server/rest/instance.py	47.27% <100%> (+0.97%)	⬆️
server/models/instance.py	24.27% <20.51%> (+24.27%)	⬆️
server/constants.py	73.33% <75%> (+0.91%)	⬆️

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