Allow vhost ssl cert andn key inheritance from http section

[jeffmccune]

Without this patch the SSL cert and key are always configured within vhost
server blocks. This is a problem when using a single wildcard certificate
operating with multiple virtual host entries because the configuration results
in increased memory consumption according to the NGINX documentation:

It is better to place a certificate file with several names and its private key
file at the http level of configuration to inherit their single memory copy in
all servers.

This patch addresses the problem by making it possible to pass the boolean
value false to the ssl_key and ssl_cert parameters. When doing so, the SSL
key and certificate will be inherited from the http section of the
configuration.

References:

• http://nginx.org/en/docs/http/configuring_https_servers.html#name_based_https_servers

[wyardley]

This seems similar to #794 (though that one is not in working state right now).
Looks pretty sensible to me, and thanks for adding tests. I'm not sure what the deal is with the coveralls failure, but I've been getting those too, I'll ask around.

[bastelfreak]

thanks @jeffmccune !