Add option to suppress du checks in collectd

Summary:
adds two new options to the memsql-collectd plugin config:

SkipDiskUsage: disables all du-checks (defaults to false)
SkipColumnarDiskUsage: only disables the du for /data/columns (defaults to false)

Test Plan: invoked collectd by hand, observed expected results

Reviewers: carl

Reviewed By: carl

Maniphest Tasks: T11289

Differential Revision: http://grizzly.memsql.com/D7683