Add Advanced Tag

Advanced Option
-----------------------
There are hundreds of options in Openstack as a whole that are considered advanced or not commonly used. Marking such options would help users understand the impact of enabling, disabling, or editing such an option. In addition, placement of advanced options would be toward the bottom of sections or subsections when generated by oslo-config-generator. This would allow clarity on common options needed for general functionality, by allowing common options to be more toward the top of sections in sample configuration files.

Usage
---------

Parameter would be 'advanced' and configured via boolean value:

```
cfg.StrOpt('option1',
             default='mydefault_value',
             advanced=True,
             help='This is help text.'),
```

Would yield an output from olso-config-generator:

```
[DEFAULT]
...
 <push to bottom of section>
...
# This is help text. (string value)
# Advanced Option: modifying/enabling this option might
# have significant effects on stability or performance.
# option1 = mydefault_value
```

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