Fix to get_schema(s) to use URL prefixed with slash for vapp.

[dmichaels-harvard]

• For the vapp (portal_vapp) case, the URL for ff_utils.get_schema(s) needs to be prefixed with a slash; maybe true in general but nervous about changing that now for fear of wider breakage.
• Also, the response from vapp.get returns a webtest.response.TestResponse, which has a json property which is an object rather than a function; handling this in ff_utils.get_response_json.

[coveralls]

Pull Request Test Coverage Report for Build 6125878108

• 6 of 7 (85.71%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.003%) to 77.782%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
dcicutils/ff_utils.py	5	6	83.33%
<!--	Total:	6	7	85.71%	-->

Totals
Change from base Build 6113817551:	-0.003%
Covered Lines:	8255
Relevant Lines:	10613

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💛 - Coveralls