digital_ocean: don't run update_attr if wait=no

Sometimes when using digital_ocean with wait=no I get the error "No ip is found".  But with wait=no I wouldn't expect there to be any IP, that gets allocated later.   However, looking at the code, it turns out that with even with wait=no it waits up to 10 seconds for an IP to be allocated.  We could wait longer, but with wait=no that seems like the wrong choice; it's easy enough to grab an IP later with a wait=yes command.

To make this change I removed the call to update_attr in @classmethod add.  An add is always followed by an ensure_powered_on which will do the update_attr if wait=yes.   It would be possible to instead do a call to update_attr with no retries and ignore the errors but I figured it would be better to be consistently not return an IP than to sometimes return it and sometimes not.   Inconsistent behaviour makes debugging deployment scripts very difficult.