Add --allow-root to all wp-cli calls

wp-cli now requires --allow-root in order to run as the root user due to security concerns (see [wp-cli/wp-cli#982](https://github.com/wp-cli/wp-cli/issues/982) and [wp-cli/wp-cli#973](https://github.com/wp-cli/wp-cli/pull/973)).  I've added this flag to all uses of wp-cli in vvv-init.sh.