fix SwitchPeers validation

[karenetheridge]

• presently, the SwitchPeers validation fails when there is a gap in the rack layout -- necessitating a 'blank' hardware product to be inserted
• also, (per Jeff) "it will need some new logic because it won't work with the new 4u servers cabling; -- well I think we need a way to have some sort of custom logic based on rack type; manta cabling should always be the same; that validation actually needs to be smarter too; becuase it only just gets the ports and validates the peer ports and thats it"

[karenetheridge]

@perigrin We need a spec for this before Jeff leaves.

[perigrin]

@jemershaw if you get a chance to brain dump here

[karenetheridge]

A better understanding of the problems with this validation is necessary for rewriting it as part of #943.