Remove the ca_cert_store option

This option is intended to pass to the client a store containing the
accepted CA certificates to be used when checking the validity of the
server certificate. But this store is an OpenSSL specific object, and it
can be used only with libraries that directly support these OpenSSL
objects. In addition this OpenSSL object doesn't provide any mechanism
to extract the contained certificates. This blocks any attempt to use an
HTTP library other than the Ruby standard one. To avoid this issue this
patch removes the ca_cert_store option. The ca_cert_file option is
preserved, so CA certificates can still be provided using a file.

Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>