Reduce code duplication in ACLCertificateData::parse() (#1242)

Reduce code duplication in ACLCertificateData::parse() (#1242)

ACLs ca_cert and user_cert already prohibit key changes. And
server_cert_fingerprint ACL only supports one option name spelling, so,
while "-sha1" is not, technically, a "key", it still cannot be
"changed". Adjust SetAclKey() (added in recent commit 4a3b853) and reuse
it to implement those existing "key change" checks.