Fix test failure on Mac OS X

The code returns an error if the requested vbuckets isn't a
power of two

Change-Id: Idd728478db0f66f244c278ef0718e7d167f94f57
Reviewed-on: http://review.couchbase.org/27739
Tested-by: Trond Norbye <trond.norbye@gmail.com>
Reviewed-by: Matt Ingenthron <matt@couchbase.com>