Handle 'infinite' and 'unknown' capacity in CapacityWeigher

This patch updates CapacityWeigher to transform 'infinite' and
'unknown' capacity reported by back-ends to float('inf').  They
are considered the same for sorting purpose.

Change-Id: Ic8f811227c9937c4e09eb8b77457ed7e9231be4e
Merge "Handle 'infinite' and 'unknown' capacity in CapacityWeigher"