Steps to reproduce:

Original issue: see https://github.com/vrogach2020/mdm-sandbox/pull/1

Given the following setup: a Cartridge application + tarantool/crud + tarantool/migrations + integration tests in luatest, an integration test trying to call crud.insert('test', {...}) fails with error Failed to insert test, error: Insert: Space "test" doesn't exist, although the space is initialized from migrations and does exist at the time of call, which is checked beforehand with a code like:

--wait until migrations are applied
    for _, server in pairs(helper.cluster.servers) do
        --spaces may be created with a slight delay on replicas
        helper.cluster:retrying({ timeout = 1 }, function()
            t.assert_not(server.net_box:eval('return box.space.test == nil'), server.alias)
        end)
    end

    helper.cluster:wait_until_healthy()

CRUD checks the space existence before performing operations on it using internal vshard connections, the simplified code looks like :

_, replicaset = next(vshard.router.routeall())
replicaset.master.conn.space[space_name]

Investigated by @rosik:
Vshard connection is established before migrations, and until the first insert, there are no other communications. Thus crud uses outdated schema and the request fails.

The suggested W/A is explicitly calling ping() for forcing schema update:

helper.cluster.main_server.net_box:eval([[
        local vshard = require('vshard')
        for _, r in pairs(vshard.router.routeall()) do
            r.master.conn:ping()
        end
    ]])

Actual result

CRUD may fail to work with spaces if vshard schema in internal connections is outdated, returning a fake "space doesn't exist" error

Expected result

CRUD doesn't fail to work with spaces when spaces actually exist

Proposed solution

1. @racktear: do not check for space existence at all.
   May lead to errors when attempting to use the space format/metadata.

2. Do not use vshard connections for checking the space existence.
   Probably requires the cluster DDL to be implemented