mon: add key[=value] ... to osd pool create

With the introduction of the erasure code pool, arguments to be
interpreted depending on the pool type must be introduced.
For instance the erasure code pool loads a plugin at run time will
use easure-code-k=10 to split each object in 10.

The arguments are described as

  name=args,type=CephString,n=N,req=false

If key=value it is stored in the new args data member of pg_pool_t
as args[key] = value, otherwise the value is the empty string.

http://tracker.ceph.com/issues/6113 refs #6113

Signed-off-by: Loic Dachary <loic@dachary.org>