[Ha Guide] Refresh minimum hardware requirements
[Ha Guide] Refresh minimum hardware requirements

Borrowed numbers from http://docs.openstack.org/developer/tripleo-docs/environments/environments.html#minimum-system-requirements

Change-Id: Ia881bcc10ebc13f10d69553d906799f18c873145

openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals