rgw: add exclusive flag to set_as_default()

rgw: add exclusive flag to set_as_default()

this dodges the race in RGWRealm::create() and RGWZoneParams::create()
that decides whether to set the new object as a default. by calling
set_as_default() with exclusive=true, it will fail with EEXIST if a
default is already set

it also fixes an issue with 'realm pull' on a secondary zone, where a
'default' zone may be created but never actually set_as_default()

Signed-off-by: Casey Bodley <cbodley@redhat.com>