Separate sampling code from vdsm.utils

vdsm.utils should only include helper functions which could be shared
different modules. Sampling is a separate function, so it should be
moved out of utils. It could resolve the problem of circular import.

Change-Id: I1de4c81b3cf202f9f548a59b86ef0c8a97c2af19
Signed-off-by: Mark Wu <wudxw@linux.vnet.ibm.com>
Reviewed-on: http://gerrit.ovirt.org/13838
Reviewed-by: Dan Kenigsberg <danken@redhat.com>