In test `test_named_hot_reload` there is no reconfiguration to
uuid_as_key identification_mode in the end of the test. This
test negatively influences other tests which expect uuid_as_key
identification_mode.

Needed for tarantool#493

NO_DOC=test

The function `assert_server_no_alerts` is moved from
`persistent_names_test` into `test/luatest_helpers/asserts.lua`.
This change has been made in order to use this function in subsequent
commit in `test_alerts_for_non_vshard_config_template`. Also some usages
of this function are changed in `persistent_names_test` with `asserts`
module.

Needed for tarantool#493

NO_DOC=test

This test expected alert for non-vshard replica to be shown, which is
incorrect behavior and will be fixed in the subsequent commit.

Needed for tarantool#493

NO_DOC=test

Before this patch vshard throwed alerts for replicas, which are not in
its config. E.g. disconnected CDC replica caused `UNREACHABLE_REPLICA`
alert. This patch fixes it by skipping unknown replicas.

Closes tarantool#493

NO_DOC=bugfix

This test checks scenario when replica disconnects from replicaset
in named vshard config. In this situation two alerts should be thrown:
`UNREACHABLE_REPLICA` and `UNREACHABLE_REPLICASET`. It is necessary to
add this test in order to reduce untested places in the codebase. Also
a function `info_assert_alert` is removed from `router_2_2_test` since
there is a more appropriate function in `asserts` module.

Follow-up tarantool#493

NO_DOC=test