Install Guide: Fix markup in heat-install

It is <systemitem class="service">

Change-Id: I4137d069000d31f070d389492527c66b1fb01877
backport: havana
(cherry picked from commit 6cbe45690e84a2f4522185f9109bd05a245679e1)
Fixes install Networking Services -- keystone endpoint-create issue

backport: havanna

Change-Id: I892c5ea4505dc003e83d13445f75fb26edbedbf4
Closes-Bug: #1246751
(cherry picked from commit c091c9002292560f667003aa9f618b791b241f1f)
Merge "Fixes install Networking Services -- keystone endpoint-create issue" into stable/havana
Merge "Install Guide: Fix markup in heat-install" into stable/havana