Added a helper function to raise warnings until a specified salt version.

The helper raises a warning, by default, a ``DeprecationWarning``, until the provided ``version_info`` is matched, after which, a ``RuntimeError`` will be raised to remember the developers to remove the warning.
**ALL** Salt `DeprecationWarning`'s should be shown **once**.
Replaced occurrences of `warnings.warn` with `salt.utils.warn_until`.
Allow `salt.utils.warn_until()` to not issue the warning because we're only after the `RuntimeError` behaviour.
Marked specific places which need code removal on salt >= 0.18