add option to execute zapp without deploy

now `execute` command has an optional `--container` argument, rather than required
if `zpm execute zapp_path.zapp` is invoked the zapp will be executed directly without deploying
if `zpm deploy zapp_path.zapp --execute` is invoked zapp will be deployed and then executed
if `zpm execute zapp_path.zapp --container container_name` is invoked an already deployed zapp will be executed

I think the last one should become `zpm execute swift://./container_name/zapp_name.zapp` instead, but lets leave it for now