Allow floating point value for dispersion_coverage

For systems with very large numbers of partitions, 1% dispersion
coverage may simply be too much/take too long. This fix allows <1
values to be used for dispersion_coverage.

DocImpact

Change-Id: I5ed35b69754d55a410e66e658b3854de57c7666b
Merge "Allow floating point value for dispersion_coverage"