rgw: add erasure_code_profile configuration

If erasure_code_profile is present at the same leve as ec-data-pool, it
is used to override the default hard coded profile.

Signed-off-by: Loic Dachary <loic-201408@dachary.org>
Merge pull request #104 from dachary/wip-8933-rgw-erasure-code-profile

rgw: add erasure_code_profile configuration