Allow Express to implement SR-IOV

[busterswt]

This PR adds some bits to Express to allow the user/operator to implement SR-IOV on one or more hosts using host/group vars. There are some caveats that should probably be called out in the README, and I'll try to update them while this is reviewed.

[mmccarre]

Few clean-up notes, questions, and possibly an invalid json payload for Neutron when SRIOV is not enabled.

[mmccarre]

Add comment header since previous one was modified to Pre-Flight checks

[xagent003]

Blocking issue: The VFs need to be persisted before hostagent is started. Or specifically, before pf9-neutron-sriov-agent is started. Otherwise things break after reboot.

On a customer, it was observed that the hosts were rebooted. When it came up, hostagent was started, which starts sriov-agent. At this time, no VFs were configured on the NIC. sriov-agent initializes some runtime state, and detected that NIC as having no VFs. Afterwards, per /var/log/messages, I saw the script /pf9-virtual-functions.sh being invoked and setting /sys/class/net/$1/device/sriov_numvfs

So those VFs were not monitored for changes later, when they were configured to VMs and neutron ports.

This can be fixed in 3 ways:

1. Properly persisting the numvfs in /etc/network/interfaces (Ubuntu) or in ifup scripts for the NIC (RedHat/Centos). This is how the official Openstack docs say: https://docs.openstack.org/neutron/rocky/admin/config-sriov.html

2. We can restart pf9-neutron-sriov-agent again in the script after setting numvfs parameter

3. Change systemd service dependencies so pf9-sriov-vf-manager.service is started BEFORE pf9-hostagent.

Since this isn't committed, this needs to be changed whereever we're hosting it anyone else already using SRIOV