Add support for elliptic curve based certificates

[elsloo]

Currently Traffic Router has limited support for certificate types, and it would be preferable if we could support elliptic curve based certificates. It would also be nice to have the ability to generate self-signed certificates of this type in addition to the existing RSA certificate type.

[mitchell852]

@rawlinp any thoughts on this?

[rawlinp]

I imagine this would still be nice to have.