README: More updates for Troubleshooting section
README: More updates for Troubleshooting section

* mention DHCP logs and tcpdump
* clarify mention Red Hat systems
* small tweaks

Change-Id: I68713f278d0a132588edcf09d338e7c7d9891fb6

openstack/ironic-inspector,rdo-management/ironic-discoverd,rdo-management/ironic-discoverd,openstack/ironic-inspector