1) Added comments to VPC/VPCService/VPCManager interfaces
2) Moved VPC offering related methods from VpcService to the new interface - VpcProvisioningService
3) Fixed static nat creation in the VPC - used to result in NPE due to invalid method referencing while obtaining VPC VR information
The new policy is:
1. Generate a random IP.
2. Find the next available IP, start from the generated IP.
3. If we cannot find an available IP after certain times(10000 by default,
network.ipv6.search.retry.max) retry, give up.