Add API schema for v3 flavor_access API

By defining the API schema, it is possible to separate the validation
code from the API method. The API method can be more simple.
In addition, a response of API validation error can be consistent for
the whole Nova API.

Partially implements blueprint nova-api-validation-fw

Change-Id: Ia3b4fa951c2f3867685ea6a619a39ac47dc4c3e6
Add API schema for v3 extended_volumes API

By defining the API schema, it is possible to separate the validation
code from the API method. The API method can be more simple.
In addition, a response of API validation error can be consistent for
the whole Nova API.

Partially implements blueprint nova-api-validation-fw

Change-Id: If0ebee7a8661baae9eea1e7ce586695b9e575afc
Cleanup the flake8 section of tox.ini

* Stop ignoring F403 as we have no cases of it anyway
* Cleanup the comments to make it clear what is ignored on purpose and
  what is ignored until we clean up the code.

Change-Id: I2500e2db70b9c71fbed781d8f7b7c80eee45140f
Merge "Cleanup the flake8 section of tox.ini"
Merge "Add API schema for v3 extended_volumes API"
Merge "Add API schema for v3 flavor_access API"
Attribute snapshot not defined in libvirt/config.py

The attribute snapshot was not defined (W0201).

Change-Id: Id864685507cee52a91213b3603044b24fbd117e3