Install Guide: Add table documenting reserved UIDs

Add a table documenting the reserved UIDs that are used by
OpenStack.
This is information that belongs IMO into an appendix, let's make
it specific for each distribution.

backport: none
Change-Id: I66f861b4ad84231af5053826dbc47df4d91bc0b9
Closes-Bug: #1239879
Merge "Install Guide: Add table documenting reserved UIDs"