(PUP-1165) Perform validation on desired triggers not current

The `enabled` and `index` trigger parameters are read-only, so they will be set
in the `current` trigger, but are not allowed in the `desired` trigger.
Previously, the `translate_hash_to_trigger` method performed validation based
on whether it was passed a `current` or `desired` trigger.

This commit moves the validation logic to the `validate_trigger` method, which
is only ever called with `desired` trigger values. It also allows the `index`
and `enabled` keys to be present in a trigger. This isn't an issue because they
are ignored for the purposes of determining if two triggers are the same.

The reason for this change, is because ultimately we don't want the
`translate_hash_to_trigger` method to mutate the `current` trigger, and this
commit makes it so that the `index` and `enabled` keys can be in the
`puppet_trigger` but will be ignored.
(PUP-1165) Ignore scheduled_task trigger start_date when unspecified

Previously, if a scheduled_task resource was created in the past, and
the desired value of the resource's trigger did not specify a
`start_date`, then puppet would think the resource was out of sync,
and attempt to delete the old trigger, and add the new one. This
process would repeat once per day, leading to spurious 'triggered
changed' messages, and reports.

This commit modifies the scheduled_task provider so that if the
desired value does not specify a `start_date`, then it will
effectively be ignored when comparing the current and desired
triggers.
(maint) Remove magic behavior

Previously, the `#triggers` method on the scheduled task provider would
magically return single element or an array of multiple elements.

This commit changes the provider to always return an array, of possibly a
single element.