Change create arg parsing, relax PagerDuty credentials validation

[ghost]

See #39 for the reasons for the create args change; the docs were correct, the code was slightly deficient.

Also removed some duplicated code left over from recent refactoring.

[Sarah-E-Greene]

Looks fine to me.

[jessereynolds]

is this good to go into a 1.2.0 diner release at the same time as flapjack 1.2 ?

[ghost]

Yup, should be fine. A slight expansion in capabilities, it now matches what I'd thought it was doing when I wrote the docs.