conpaas-docs: expand frontend introduction
conpaas-docs: expand frontend introduction

Mention that director and frontend can (but don't have to) be installed on the
same host.


git-svn-id: 3c739b8a310a10be68afd4fdd7f3d450bf1457ba@4905 ef35cc4a-2640-4bbf-98bc-65920db7d8ae

ema/conpaas,ema/conpaas,ema/conpaas,ema/conpaas,ema/conpaas