Install and configure a compute node for Red Hat Enterprise Linux and CentOS in nova

[Jiang]

openstack pike requires qemu version equal or greater than 2.1, but the doc https://docs.openstack.org/nova/pike/install/compute-install-rdo.html did not mention this. I think it should add a tips to the doc.
And centos default qemu version is 1.5. It is very difficult to upgrade qemu on centos. So, is it possible to add a tutorial about how to upgrade qemu?

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-----------------------------------
Release: 16.0.1.dev6 on 2017-09-07 00:23
SHA: e06912594ff092ad97c34712e8d4d9194d3b88f7
Source: https://git.openstack.org/cgit/openstack/nova/tree/doc/source/install/compute-install-rdo.rst
URL: https://docs.openstack.org/nova/pike/install/compute-install-rdo.html