Wait for celery to return response to request to solve specification

[soapy1]

Fixes #615 and #761

Description

The GET /specification endpoint is meant to allow users to specify some conda environment configuration (packages, channels, etc) and return the set of packages for the solved environment.

Previously, this endpoint was broken

$ curl http://localhost:5000/conda-store/api/v1/specification/
Internal Server Error

With this change, users can use the endpoint

$ curl http://localhost:5000/conda-store/api/v1/specification/
{"solve":[{"id":117414,"channel_id":2,"build_number" .... 

Pull request checklist

• [x] Did you test this change locally?
• [x] Did you update the documentation (if required)?
• [x] Did you add/update relevant tests for this change (if required)?

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