The quota handling for the simple driver (used by GCE) only handles
max-servers, but even so, it still didn't take into consideration
currently building servers. If a number of simultaneous requests
were received, it would try to build them all and eventually return
node failures for the ones that the cloud refused to build.
The OpenStack driver has a lot of nice quota handling methods which
do take currently building nodes into account. This change moves
some of those methods into a new Provider mixin class for quota
support. This class implements some handy methods which perform
the calculations and provides some abstract methods which providers
will need to implement in order to supply information.
The simple driver is updated to use this system, though it still
only supports max-servers for the moment.
Change-Id: I0ce742452914301552f4af5e92a3e36304a7e291
9e9a5b9bfd