Specify "Horizontal" scalability

Let's add some meaning to a vacuously true statement - everything out there claims to be scalable. In OpenStack's case, the statement actully has meaning.

Change-Id: I2a9ca529a72c88eda97b4ac889528b3792a98f0f
backport: none
Closes-Bug: #1317190
Merge "Specify "Horizontal" scalability"