BUG: Index may ignore specified datetime/timedelta dtypes
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

`Index` ignores specified `dtype` if it is `datetime64` (normal and tz) or `timedelta64`. This PR makes it consistent with `DatetimeIndex` and `TimedeltaIndex`.

```
pd.Index([1, 2, 3], dtype='datetime64[ns, US/Eastern]')
# Index([1, 2, 3], dtype='object')
pd.Index([1, 2, 3], dtype='datetime64[ns]')
# Index([1, 2, 3], dtype='object')
pd.Index([1, 2, 3], dtype='timedelta64[ns]')
# Int64Index([1, 2, 3], dtype='int64')
```

Also, fixed `MultiIndex.get_level_values` not to pass unnecessary `tz` and `freq`.

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13981?src=pr) is 85.27% (diff: 100%)

> Merging [#13981](https://codecov.io/gh/pydata/pandas/pull/13981?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will decrease coverage by **0.01%**

``` diff
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- Hits          42818      42810     -8   
- Misses         7388       7395     +7   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [29d9e24...a922aef](https://codecov.io/gh/pydata/pandas/compare/29d9e24f4c778b0c9ebe9288bfc217808d2c6edb...a922aefa1b40178616d890ca2cfe3f897cb497e6?src=pr)
 thanks!
 @sinhrks Is it correct that you introduced the `tz` keyword in `Index` in this PR? 
Personally I don't think this is needed (it is also not documented).

To be clear, the fact that `dtype` is no longer ignored in `Index` is a nice bug fix! My comment is only about the tz keyword.
 tz is NOT in the Index signature but you can pass kwargs generally
 @jorisvandenbossche prior to this PR, passing `tz` kwd coerces to `DatetimeIndex` in some cases. This PR makes it consistent.
- https://github.com/pydata/pandas/blob/master/pandas/indexes/base.py#L267
 `tz` was previously passed through to  when you already had datetime data, eg:

```
In [10]: pd.Index([datetime.datetime(2012,1,1), datetime.datetime(2012,1, 2)], tz="Europe/Brussels")
Out[10]: DatetimeIndex(['2012-01-01 00:00:00+01:00', '2012-01-02 00:00:00+01:00'], dtype='datetime64[ns, Europe/Brussels]', freq=None)
```

But the consequence of this PR is that for example the following now also works:

```
In [82]: pd.Index([1, 2], tz="Europe/Brussels")
Out[82]: DatetimeIndex(['1970-01-01 00:00:00+01:00', '1970-01-01 00:00:00+01:00'], dtype='datetime64[ns, Europe/Brussels]', freq=None)
```

I personally don't think this should work like that .. (a bit too much magic). 
