Add Related Options

Related Options
---------------------
Often times configuration of an option has implications for other options. Adding a 'related_options' parameter to list out all impacting options via an array, would help users understand the impact of enabling, disabling, or even adjusting a configuration option.

Usage
---------

Parameter would be 'related_options' and configured similar to 'choices':

```
cfg.StrOpt('option1',
             default='mydefault_value',
             related_options=('option2', 'option3', 'option4'),
             help='This is help text.'),
```

Would yield an output from olso-config-generator:

```
[DEFAULT]
# This is help text. (string value)
# Related Options: option2, option3, option4
# option1 = mydefault_value
```

References
---------------
[1] Cross Project Spec - https://review.openstack.org/#/c/295543