vstart: set a sensible default for ruleset-failure-domain

Set ruleset-failure-domain=osd so that

  ./ceph osd pool create ecpool 12 12 erasure
  ./rados --pool ecpool put SOMETHING /etc/group

works by default. When using a vstart cluster the default failure
domain (host) won't work because all OSDs are in "localhost".

Signed-off-by: Loic Dachary <loic@dachary.org>