use notcmalloc builder for valgrind leak checking

This reverts 453d72b294af6d3db1ae7a212ba9b43209d69f26.  It turns out that
tcmalloc defeats the leak checking in valgrind.

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #53 from ceph/wip-leaks

use notcmalloc to fix leak checking