Provider network implementation for NVP plugin.

blueprint nvp-provider-net

Implements the provider network extension support. The list of valid network
types has been updated to reflect the types supported by the nvp plugin.
This was necessary otherwise validation would have always failed.
Multiple logical switches might be associated with a quantum network; the
first logical switch will always have the same id as the quantum network.
Also now raises exception when port limit on overlay network is reached.

This patch also adds a check for the maximum number of ports on 'standard'
overlay networks, and performs some code refactoring for improving
maintanability. For instance the NVPCluster class has been moved into its own
module.

Change-Id: Ib26d327daf748cfcba9ca74e8dc2e8e89c676c2e
Merge "Provider network implementation for NVP plugin."