Doc: Remove prerequisite: Ubuntu
Doc: Remove prerequisite: Ubuntu

There is no single action that is based on debian/ubuntu.
Only pip and Linux tools are used that should be the
same for all platforms.

Trivial-fix
Change-Id: Iecb7474b2bb90ebb4c3965b37ab3a893a8ee1229

scality/manila,vponomaryov/manila,vponomaryov/manila,NetApp/manila,bswartz/manila,openstack/manila,NetApp/manila,openstack/manila,bswartz/manila,scality/manila