Make slaveOkay(true)->slaveOkay(false); etc also work.

With slave_okay being true or absent, the RP is switched to SECONDARY
PREFERRED but only if the current configured RP is PRIMARY - so that
other read preferences are not overwritten. As slaveOkay really only
means "read from any secondary" that does not conflict.

With slave_okay being false, the RP is switched to PRIMARY. Setting it
to PRIMARY when it already is PRIMARY doesn't hurt.