MINOR: pools: preset the allocation failure rate to 1% with -dMfail

MINOR: pools: preset the allocation failure rate to 1% with -dMfail

Using -dMfail alone does nothing unless tune.fail-alloc is set, which
renders it pretty useless as-is, and is not intuitive. Let's change
this so that the filure rate is preset to 1% when the option is set on
the command line. This allows to inject failures without having to edit
the configuration.