osd/OSDMap: do not create erasure rule by default

If we do, we will require the v2 feature bit from clients.

We could only include feature bits for rules that are actually referenced
by pools, but for now making the user create the rule is simpler.  There is
no need to create this rule ahead of time.

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #1132 from ceph/wip-erasure-rule

osd/OSDMap: do not create erasure rule by default

Reviewed-by: Greg Farnum <greg@inktank.com>
Reviewed-by: Loic Dachary <loic@dachary.org>