vic-machine: detect and address subnet segregation between VC and ESX hosts

[hickeng]

Story As a viadmin I want vic-machine to fully validate my deployment options against the infrastructure As a viadmin I follow VMware best practice and place vCenter and ESX hosts in separate subnets

Details The vic-machine create --management-network-gateway supports specifying additional subnets that can be reached via the management interface, however knowing that it's needed, finding how it's specified, and getting it right it are all awkward. There is also the --asymmetric-routes option but that's brute force rather than finesse and I've also seen it be ineffective in one case where explicitly routing worked (never tracked down why).

vic-machine should inspect the subnets of the ESX hosts in the target cluster and correctly populate the additional routing data for the management interface.

Acceptance

• [ ] There is no need to specify additional subnet options under normal circumstances even when ESX and VC are on separate subnets.
• [ ] vCenter is accessed via the management interface even if VCH management IP is not on the same subnet

@malikkal @pdaigle