osd: add default_crush_rule for erasure pools

The osd_pool_default_crush_rule option is replaced with
osd_pool_default_crush_rule_replicated and
osd_pool_default_crush_rule_erasure is added.

For backward compatibility purposes, all are initialized to
OSD_POOL_DEFAULT_CRUSH_RULE_UNSET. If osd_pool_default_crush_rule is set
a deprecation warning is printed. If osd_pool_default_crush_rule and
osd_pool_default_crush_rule_replicated are set,
osd_pool_default_crush_rule_replicated takes precedence.

Signed-off-by: Loic Dachary <loic@dachary.org>