Add release version to config reference options

The continuously published configuration reference contains tables with information about all configuration options in OpenStack.

Currently, these tables are updated from master, and always have the latest information in them.

However, there continues to be users of older releases who still need reference information for their configuration options.

The toolchain used to generate the tables is able to operate on any git repository checkout, meaning it can also generate historical tables.

This blueprint proposes to automatically provide some form of release information with the configuration option by comparing the options generated from two different repositories.

For example
* an option that appears in a Havana repository, but does not appear in an Icehouse repository was deprecated in Havana.
* an option that does not exist in Grizzly, but appears in Havana is a new option to Havana

These sort of cases should then be communicated to the reader. A selection between three potential forms for this information are envisaged:
1. tables are updated with an additional column that provides release information (eg "available from Icehouse onwards", "deprecated in Essex")
2. a table of "new and removed" options between Icehouse and Juno is added as an appendix
3. the configuration reference is branched for the Juno release, with different versions of the tables in each