Allow for multiple signing keys in metadata documents if entity has Cert...

Because of [HeartBleed](http://heartbleed.com) we will need to renew the private key for EB. However to do that we first need to let the SPs know that there will be another key for signing.

With this patch, if an entity has CertData2 and/or CertData3 defined, they will appear in the metadata document as signing keys.