remove flag --allow-root when possible with WP-CLI

Few times the flag --allow-root is used with WP-CLI when :

• root is used to execute, but it is probably not required
• user attribute is defined in exec

Examples :

• https://gitlab.adullact.net/adullact/puppet-wordpress/blob/master/manifests/core.pp#L187
• https://gitlab.adullact.net/adullact/puppet-wordpress/blob/master/manifests/core.pp#L200

A review about this is needed.