Fix luacheck warnings

Closes #178

Author: ligurio

test/cluster-py/master.lua

@@ -1,5 +1,5 @@
 #!/usr/bin/env tarantool
-os = require('os')
+local os = require('os')
 box.cfg({
     listen              = os.getenv("LISTEN"),
     memtx_memory        = 107374182,

unit/suites/box.lua

@@ -1,5 +1,5 @@
 #!/usr/bin/env tarantool
-os = require('os')
+local os = require('os')
 
 require('console').listen(os.getenv("ADMIN_PORT"))
 box.cfg{