Fixed minor letters error

ee/cli/plugins/stack.py

@@ -60,8 +60,8 @@ class EEStackController(CementBaseController):
             EERepo.add(ppa='deb http://repo.percona.com/apt '
                        + EEVariables.ee_platform_codename + " main")
             EERepo.add_key('hkp://keys.gnupg.net', '1C4CBDCDCD2EFD2A ')
-            
+
-            chars = ''.join(random.sample(string.letters, 8))
+            chars = ''.join(random.sample(string.ascii_letters, 8))
             EEShellExec.cmd_exec("echo \"percona-server-server-5.6 "
                                  "percona-server-server/root_password "
                                  "password {chars}\" | "