Add support for multiple platforms in OpenShift.
Changes span both the broker and the node.

Add a setting named 'NODE_PLATFORMS' to broker.conf file that lists available node platforms.
Add a optional attribute named 'platform' to cartridge manifests.
Add a optional 'Required' attibute for the 'Scaling' key in the cartridge manifest.

Broker queries nodes of all platforms when building its cartridge cache.
Broker does not allow group overrides for cartridges of different platforms.
Broker uses the cartridge platform to filter nodes based on the 'kernel' fact.

Node handles the case when a web proxy cartridge is not colocated with a web cartridge.