rancid-all should accept OS type (or type list?)

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You might have different OSes in the same group (e.g., at Arista we have dlink 
and arista in the switches group), and so need to use different commands on the 
different devices.

One idea is ./all switches arista script

although you might want to be able to specify multiple OSes, if they're 
compatible, e.g.,

./all switches arista,cisco script

Original issue reported on code.google.com by fen...@gmail.com on 21 Nov 2010 at 3:02