Fail if rabbitmq proposal cannot be found on creation

RabbitMQ is now a hard dependency of glance, since 29f4cc0a.
Use "glance-manage db sync" instead of "glance-manage db_sync"

This was changed upstream to be compatible with the nova-manage
commands.
Merge pull request #226 from vuntz/db-sync

Use "glance-manage db sync" instead of "glance-manage db_sync"
Merge pull request #225 from vuntz/no-optional-rabbit

Fail if rabbitmq proposal cannot be found on creation