added default deployment descriptor option

added a configuration option called default_deploy which specifies a filename where TorqueSpec should look for a default deployment descriptor.  If the file exists then this default deployment descriptor is used for all specs except for those which explicitly define a deploy statement within their describe block.  Specs which default to using the default_deploy descriptor will not need to undeploy and redeploy before they run.  The default_deploy configuration option defaults to "default_deploy.yml".