mlab-ns should apply a weight to sites based on switch utilization

[stephen-soltesz]

Today mlab-ns does not take advantage of the relative capacity differences at sites, e.g. 10g vs 1g uplinks.

mlab-ns should be able to apply a weight to individual sites based on some monitored criteria, like switch utilization. For example, if a 10g site is only 10% utilized and a 1g site is 90% utilized, then the 10g site should have a 90% probability of being selected and the 1g site only a 10% chance.

[stephen-soltesz]

Assigning to @pboothe based on in person discussions -- can be reassigned as necessary.