Lift the restriction on choices for `cpu_model_extra_flags` config attribute

[Kashyap Chamarthy]

In the patch[*] that introduced the Nova configuration attribute
`[libvirt]/cpu_model_extra_flags`, we have restricted the choices to be
only 'PCID' -- to alleviate the immediate guest performance degradation
as a result of applying the "Meltdown" CVE fixes.

Now remove that restriction to allow adding and removing multiple CPU
flags, making way for other use cases.

Use cases:

  - Ability to use 1GB huge pages with Haswell model as one use case for
    extra flags:

        cpu_model=Haswell-noTSX-IBRS
        cpu_model_extra_flags="pdpe1gb"

  - Nested Virtualization -- an operator can specify the Intel 'vmx' or
    AMD 'svm' flags in the level-1 Nova guest.

[*] http://git.openstack.org/cgit/openstack/nova/commit/?h=master&
    id=6b601b7 -- libvirt: Allow to specify granular CPU feature flags

A potential example of specifying multiple CPU feature flags
------------------------------------------------------------

If you specify:

    [libvirt]
    cpu_mode=custom
    cpu_model=IvyBridge
    cpu_model_extra_flags="+pcid,-mtrr,pdpe1gb"

Then, Nova should generate the below XML:

    <cpu match='exact'>
      <model fallback='forbid'>IvyBridge</model>
      <vendor>Intel</vendor>
      <feature policy='require' name='pcid'/>
      <feature policy='require' name='pdpe1gb'/>
    </cpu>

The +/- for individual flags in nova.conf will be optional. If nothing
is specified, assume '+'.

You might ask: "Why would you want to remove a CPU flag though?" One
scenario for that is: An Operator wants to generate a baseline CPU
config. And a certain CPU flag is causing performance issue or other
nuisance, and if the Operator isolated the problem to _that_ specific
CPU flag, then she may want to remove the flag.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/563926

[Matt Riedemann (mriedem)]

This isn't a bug, it's already tracked with a blueprint so just use that:

https://blueprints.launchpad.net/nova/+spec/libvirt-cpu-model-extra-flags

[Kashyap Chamarthy (kashyapc)]

Okay. I wasn't tracking it as a "bug" per se, but mostly as a tracking item / remaining portion of the Blueprint / RFE. Thought reading the text makes it obvious.

[Kashyap Chamarthy (kashyapc)]

Not to belabor this: FWIW, as you know, we intentionally restricted the original feature. So now we're just fixing that "bad" implementation. So personally, I'd prefer to track this item in this bug too. But if you really insist, I can let it go.

[Matt Riedemann (mriedem)]

I insist.