DocumentRoot: fallback to SiteName like templates

Templates already fallback to SiteName before site id.

This attemps to be consistent with them, and avoid having to explicitly specify
the DocumentRoot, when the template already does the proper inference.

apache/vhosts/standard.sls

   - apache
 
 {% for id, site in salt['pillar.get']('apache:sites', {}).items() %}
-{% set documentroot = site.get('DocumentRoot', '{0}/{1}'.format(apache.wwwdir, id)) %}
+{% set documentroot = site.get('DocumentRoot', '{0}/{1}'.format(apache.wwwdir, site.get('ServerName', id))) %}
 
 {{ id }}:
   file: