router: skip failover health check during initial configuration

This commit fixes the flakiness of the failover/cluster_changes
test, which was caused by the commit 9fc976d ("router: calls
affect temporary prioritized replica"). It started to check
`replica.net_sequential_ok` during `up_replica_priority`. However,
during the configuration of the instance, `net_sequential_ok` is 0,
so when failover fiber didn't manage to ping the instance
(e.g. connection have not been created yet) router throwed
SUBOPTIMAL_REPLICA alert.

Let's disable health checkers during router configuration
(when prioritized replica is not set at all).

Closes #495

NO_DOC=bugfix
NO_TEST=<covered by failover/cluster_changes>

vshard/replicaset.lua

@@ -513,7 +513,8 @@ local function replicaset_up_replica_priority(replicaset)
             -- Failed to up priority.
             return
         end
-        if replica:is_connected() and replica.net_sequential_ok > 0 then
+        local is_healthy = replica.net_sequential_ok > 0
+        if replica:is_connected() and (is_healthy or not old_replica) then
             assert(replica.net_sequential_fail == 0)
             replicaset.replica = replica
             assert(not old_replica or