build_tests gains a require_ssl argument which, if set to True,
makes all the loaded test suites default to 'ssl: True'.
gabbi-run will interpret a target containing 'https' as meaning
that the tests in the provided yaml should default to 'ssl: True'.
Fixes: #50
Fixes: #105
Fixes: #138
The changes here are the naive basics to get the desired behavior.
There's an existing cleanup branch on which we can clean this up
later.
