erasure-code: create default profile if necessary

After an upgrade to firefly, the existing Ceph clusters do not have the
default erasure code profile. Although it may be created with

   ceph osd erasure-code-profile set default

it was not included in the release notes and is confusing for the
administrator.

The *osd pool create* and *osd crush rule create-erasure* commands are
modified to implicitly create the default erasure code profile if it is
not found.

In order to avoid code duplication, the default erasure code profile
code creation that happens when a new firefly ceph cluster is created is
encapsulated in the OSDMap::get_erasure_code_profile_default method.

http://tracker.ceph.com/issues/8601 Fixes: #8601

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