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.
This may not be fully adequate but is a reasonable start. The main
point was to indicate the different loader method and the need
to yield the generated tests.
