luatest: eliminate luatest_helpers.lua

The luatest_helpers.lua module confuses developers because luatest
itself has the helpers.lua module as well. It is unclear which one
should be used and when because it is hard to remember what stuff
provides each of them.

Actually, luatest_helpers.lua has functions that can be safely moved to
more proper places (luatest_helpers/server.lua & instances/default.lua).

The `get_vclock` function logically belongs to server stuff, so it was
moved to server.lua and slightly changed (`eval` call was replaced by
`exec`).

The `wait_vclock` function logically belongs to server stuff as well, so
it was moved to server.lua without changes.

The `instance_uri` function logically belongs to server stuff as well,
so it was moved to server.lua and slightly changed (it was renamed into
`build_instance_uri` and redundant `instance_id` param was dropped).

The `default_cfg`, `env_cfg`, `box_cfg` functions logically belongs to
instance stuff, so they were moved to default.lua without changes.

Thus, the luatest_helpers.lua module was dropped and tests using this
module were updated.

Closes tarantool/luatest#192

NO_DOC=testing stuff
NO_CHANGELOG=testing stuff