Remove os="cirros" attribute

As there is no Cirros specific edition of the guide, and it's unlikely
there ever will be one, this note will never actually be displayed. This
change removes the attribute and rephrases the text of the node slightly.

Change-Id: I21ea1244e9e95ac7de25417a391a4909397a61aa
backport: havana
Merge "Remove os="cirros" attribute"