rgw: RGWPeriod::reflect() sets master zonegroup as default

rgw: RGWPeriod::reflect() sets master zonegroup as default

if RGWPeriod::reflect() is called right after a 'realm pull', it will
create the zonegroup objects but not set any as default.  so the next
time RGWRados is initialized, create_defaults() will add a new 'default'
zonegroup and set it as the master.  this causes a 'period update' to
find multiple master zonegroups and fail with EINVAL

reflect() now calls set_as_default(exclusive=true) on the master
zonegroup to guarantee that a default is set

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