radosgw-admin: allow 'period pull --url' without full multisite config

radosgw-admin: allow 'period pull --url' without full multisite config

to support 'period pull --remote', radosgw-admin needs a complete
multisite configuration to look up remote endpoints by name

however, 'period pull' will also accept a --url argument, which doesn't
require a working configuration. this patch splits the handling of these
two cases, so that 'period pull --url' can succeed when run on a cluster
that hasn't yet been fully configured

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