Sphinx supports various notice blocks but the oslosphinx theme
does support only warning and note. openstackdoctheme used by
openstack-manuals project supports 'important', 'tip' and 'caution'
blocks. It would be nice if oslosphinx support these notices.
It reduces confusions on contributors to both types of projects.
Change-Id: I3e1db551b05fa681520763c24e2576f9ad5379d8