Fix issues that may arise when passing invalid or unusual arguments to create-slave command

Add tests that checks that generated .tac file is valid Python script by `exec()`-uting it with mocked globals imports, such as `twisted.application.service.Application` and `buildslave.bot.BuildSlave`.

Then it checks, that `Application` was created and stored in global variable `application` (as .tac specification requires); checks that `BuildSlave` created with passed arguments and attached to application.

Add tests for using "unusual" characters in create-slave arguments and fix arised errors.

Add checks for create-slave arguments and tests for them.