Replace ca_cert_store with ca_cert

The ca_cert_store 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 replaces the
ca_cert_store option with a new ca_cert option. This new option should contain
a string with the PEM encoded certificates. 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>