MEDIUM: ssl: Added support for creating SSL_CTX with multiple certs

MEDIUM: ssl: Added support for creating SSL_CTX with multiple certs

Added ability for users to specify multiple certificates that all relate
a single server. Users do this by specifying certificate "cert_name.pem"
but having "cert_name.pem.rsa", "cert_name.pem.dsa" and/or
"cert_name.pem.ecdsa" in the directory.

HAProxy will now intelligently search for those 3 files and try combine
them into as few SSL_CTX's as possible based on CN/SAN. This will allow
HAProxy to support multiple ciphersuite key algorithms off a single
SSL_CTX.

This change integrates into the existing architecture of SNI lookup and
multiple SNI's can point to the same SSL_CTX, which can support multiple
key_types.