Fix error when auto-failover is disabled.
Fix error when auto-failover is disabled.

When auto-failover was disabled, and you tried to disable
it again, it lead to an error message. Now it will just
do nothing.

Change-Id: Iff1356c6c7137dc7a1b3c7185f56701e159bca6e
Reviewed-on: http://review.couchbase.org/7685
Tested-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>
Reviewed-by: Bin Cui <ed18693fff32c00e22495f4877a3b901bed09041@gmail.com>

daverigby/ns_server,membase/ns_server,membase/ns_server,vzasade/ns_server,membase/ns_server,t3rm1n4l/ns_server,mschoch/ns_server,daverigby/ns_server,vzasade/ns_server,pavel-blagodov/ns_server,t3rm1n4l/ns_server,pavel-blagodov/ns_server,ceejatec/ns_server,daverigby/ns_server,mschoch/ns_server,membase/ns_server,mschoch/ns_server,daverigby/ns_server,ceejatec/ns_server,t3rm1n4l/ns_server,daverigby/ns_server,t3rm1n4l/ns_server,nimishzynga/ns_server,nimishzynga/ns_server,nimishzynga/ns_server,membase/ns_server,vzasade/ns_server,mschoch/ns_server,ceejatec/ns_server,pavel-blagodov/ns_server,nimishzynga/ns_server,pavel-blagodov/ns_server,vzasade/ns_server,mschoch/ns_server,nimishzynga/ns_server,ceejatec/ns_server,membase/ns_server,ceejatec/ns_server,ceejatec/ns_server,mschoch/ns_server,t3rm1n4l/ns_server,nimishzynga/ns_server