pool create erasure : implicit ruleset creation

It improves user experience and documentation by removing the requirement to create a matching ruleset for the erasure coded pools
<pre>
    ./ceph osd crush rule create-erasure erasure-code-k=3 ecruleset
    ./ceph osd pool create ecpool 12 12 erasure crush_ruleset=ecruleset erasure-code-k=3
</pre>
can be replaced with
<pre>
    ./ceph osd pool create ecpool 12 12 erasure erasure-code-k=3
</pre>
and a new **ecpool** ruleset will be created (named after the pool name).

http://tracker.ceph.com/issues/7571