Install Guide: Consistent root command usage

The guide does not use sudo as explained in ch_basics, so remove
its usage and change prompts to #.

Cherry-Pick: https://review.openstack.org/#/c/52773/
Change-Id: I4e25e700bd070a5aca701eb2413b86e780f47192
Backport: stable/havana
Merge "Install Guide: Consistent root command usage" into stable/havana