puppet-neutron: use_ssl => true shouldn't require ca_cert parameter

When setting use_ssl => true in the neutron module, it forces you to provide a ca_file value. There are cases where you want to enable use_ssl, but not provide a ca_file: i.e. you want to enable SSL on connections to neutron, but do not want to do auth via client certs (in neutron, if ca_cert is set, it uses client cert auth and other auth methods fail.)

use_ssl => true should be required if cert_file, key_file, and/or ca_file are set, but ca_file should not be required just because use_ssl => true.