Stripe/PPE: Ensure the credentials are set on each gateway before performing API requests

[mircobabini]

…request

All Submissions:

• Have you followed the Contributing guideline?
• Does your code follow the WordPress' coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Running the migration tool from 3.0 for the subs table makes a large number of requests.
Many of them can be avoided if you still have orders for a disabled gateway w/o the credentials set.

This was already implemented for some gateway.
The support is extended to Stripe (checking the secret key only) and to PPE (checking user/pass/signature keys).

How to test the changes in this Pull Request:

1. Open an errored subscription, which has a message from the gateway after trying to connect w/o credentials
2. Apply this patch and update the subscription
3. The error will now be the message from this patch; this means no API request has been made bc there are no credentials set.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you successfully run tests with your changes locally?