mon/AuthMonitor: ensure initial rotating keys get encoded when create_initial called 2x

The create_initial() method may get called multiple times; make sure it
will unconditionally generate new/initial rotating keys.  Move the block
up so that we can easily assert as much.

Broken by commit cd98eb0c651d9ee62e19c2cc92eadae9bed678cd.

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Yehuda Sadeh <yehuda@inktank.com>