Added configration for /etc/heat/heat.conf

It fixes missing parameter in heat install documentation (api-paste.ini).

backport: havana

Change-Id: I498a57224097f2630551577f2915466d41b4e827
Closes-Bug: #1243242
(cherry picked from commit 2c4a8992376caef91f6bd05bb0c8d038dda26291)