Add support for multiple certificates

[dezeroku]

This change allows customCertificate entry to be an array of objects. Each object describes a single certificate entry, using the supported options.

The change is fully backwards compatible (object type customCertificate works exactly as it did before).

Also some functions are now async.

Tested with serverless 3.1.1

TODO:

1. not sure what to do with return values now (multiple function calls = multiple return values). These seem to be safe to ignore, but would be great if someone could confirm it
2. I was testing this with newest serverless (3.1.1) and the getCertificatePropertyOld didn't seem to be called by anything. It's likely something used in older versions? Needs to be checked and ported there too

[dezeroku]

This fixes https://github.com/schwamster/serverless-certificate-creator/issues/47

[0Lucifer0]

LGTM

[stankowalik]

+1 for this