(SERVER-39) Allow a certificate to be passed into a request via X header

This commit adds the ability for a certificate to be supplied in a web
request as the payload of a custom HTTP header, X-Client-Cert.  The
header will only be parsed for a value if the `allow-header-cert-info`
master setting is set to true.  When the certificate is retrieved, it
will be supplied as the client certificate to the Ruby request handler
stack, as a direct replacement for a certificate object that would be
typically be provided over SSL.
Not for now, no. Talked about this with @cprice404 and we were thinking that since there's a possibility that these X- header settings might be pulled out of "puppet.conf" at some point and that there was no current backward compatibility requirement for this header that we'd forgo defining a setting for this for now.

The idea of using a `def` is a good one.  I'll do that.