Incorporate feedback on Device Identity Provisioning spec. (#31)

- Renamed the spec to "Device Identity Provisioning"
- s/trust anchor/PKI root key/
- s/trust domain owner/PKI owner/
- s/certificate disablement/certificate deactivation/
- Removed intermediate certs from diagrams.
- Add language around point-in-time reliance on vendor PKIs.
- Specify that cert revocation and attestation verification are out of scope.

Signed-off-by: Jeff Andersen <jeffandersen@google.com>

Author: bluegate010