erasure-code: ErasureCode default value is a string

erasure-code: ErasureCode default value is a string

The to_int, to_bool prototypes are modified and the default values are
changed to string instead of int. If a default value is set, the profile
is modified to contain the default value.

The to_string helper is added so it can be used to set the default value
of a string, in a way that is consistent with to_int and to_bool.

http://tracker.ceph.com/issues/9589 Fixes: #9589

Signed-off-by: Loic Dachary <ldachary@redhat.com>