Install Guide: fix heat endpoints

Use the same syntax as the other services for Heat endpoints creation in
keystone.

Change-Id: Id6e04919ee154b2c1c692369eced31d69a016570
backport: havana
(cherry picked from commit 19d351ad10ac95c5e15d52c086ca73f2d2904d06)
Merge "Install Guide: fix heat endpoints" into stable/havana