fix reading of tls secret without crt or key
fix reading of tls secret without crt or key

Secrets with a CA bundle or crt+key pair uses the same internal struct
to represent them. Distinct functions verify if the proper fields are
filled to determine if the secret is correctly configured. The crt+key
reading however is checking a common field used by CA and crt secrets,
leading to a nil dereference when a certificate isn't properly
configured. This update ensures that a certificate is properly
configured. Thanks Maurilio for finding this.

Should be merged as far as v0.10.

jcmoraisjr/haproxy-ingress,jcmoraisjr/haproxy-ingress