Allow ChaCha20-Poly1305 in DTLS

GCM and CCM are modes of operation for block ciphers only. ChaCha20-Poly1305
operates in neither of them but it is AEAD. This change also enables future
AEAD ciphers to be available for use with DTLS.

Signed-off-by: Rich Salz <rsalz@akamai.com>
Reviewed-by: Matt Caswell <matt@openssl.org>