mon: osd pool create sets pg_pool_t::stripe_width

mon: osd pool create sets pg_pool_t::stripe_width

It does nothing if the pool is replicated. Otherwise it uses
osd_pool_erasure_code_stripe_width as the desired stripe width and run
it by get_chunk_size() on the erasure code plugin to get the actual
stripe_width. It will always be >= 0 to the desired stripe_width, padded
to match the alignment constraints imposed by the erasure code plugin.

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