Don't crash in graceful failover start if bad node is passed.

Change-Id: I2181390cd0fab202c07423f5be53e34522ec2ab3
Reviewed-on: http://review.couchbase.org/38903
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Keep parse_failover_args function pure.

Change-Id: I7dc6016f34ea10ff32cd065aada9bd8f206a2c1d
Reviewed-on: http://review.couchbase.org/38904
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>