Allow volumes in puppet_config containers spec

Allow volumes in puppet_config containers spec

Mounting host volumes when running containers via puppet_config already
works and is supported with docker-puppet.py. However, the validation in
yaml-validate.py does not allow it. This patch makes it allowed by the
validation.

It is sometimes necessary since some puppet modules expect to make
persistent file system changes other than just configuration data under
/etc.

In particular, ironic inspector expects to configure a http and tftp
root director with an ipxe configuration. See:
https://github.com/openstack/puppet-ironic/blob/master/manifests/inspector.pp
These changes would be lost if the value for those directories are not
mounted as host volumes.

Change-Id: Ie51c653f4c666fbaaef0ea80990e2e61f4b1353b