fix external repl switchover, add mysql-db ignore in external repl #221

Job	Run time
Build images	8s
test (GODOG_FEATURE=active_nodes.feature make test)	1m 38s
test (GODOG_FEATURE=async.feature make test)	1m 36s
test (GODOG_FEATURE=cascade_replicas.feature make test)	1m 47s
test (GODOG_FEATURE=CLI.feature make test)	1m 40s
test (GODOG_FEATURE=crash_recovery.feature make test)	1m 39s
test (GODOG_FEATURE=events_reenable.feature make test)	1m 29s
test (GODOG_FEATURE=failover.feature make test)	1m 52s
test (GODOG_FEATURE=free_space.feature make test)	1m 50s
test (GODOG_FEATURE=host_discovery.feature make test)	1m 49s
test (GODOG_FEATURE=host_management.feature make test)	1m 34s
test (GODOG_FEATURE=maintenance.feature make test)	1m 48s
test (GODOG_FEATURE=offline_mode.feature make test)	1m 28s
test (GODOG_FEATURE=priority.feature make test)	1m 33s
test (GODOG_FEATURE=readonly_filesystem.feature make test)	1m 38s
test (GODOG_FEATURE=recovery.feature make test)	2m 20s
test (GODOG_FEATURE=repair.feature make test)	1m 28s
test (GODOG_FEATURE=statefile.feature make test)	1m 36s
test (GODOG_FEATURE=switchover_from.feature make test)	1m 42s
test (GODOG_FEATURE=switchover_to.feature make test)	1m 31s
test (GODOG_FEATURE=zk_failure.feature make test)	1m 33s
test (GODOG_FEATURE=zk_maintenance.feature make test)	1m 46s
	35m 25s

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