Some internal modules have been recently copied to luatest repo [1] and
now they can be safely removed, and the corresponding functionality from
luatest can be used instead.

Affected modules:

- test/luatest_helpers/cluster.lua

[1] tarantool/luatest#271

Closes tarantool/luatest#237
Closes tarantool/luatest#269

NO_DOC=testing stuff
NO_TEST=testing stuff
NO_CHANGELOG=testing stuff