fixes #11649 - taxonomies api - add description to base.json.rabl

Adding the 'description' to the base.json.rabl so that it is displayed
when a user performs commands like 'organization list' from hammer-cli-katello.

E.g.

hammer> organization list
---|----------------------|----------------------|---------------
ID | NAME | LABEL | DESCRIPTION
---|----------------------|----------------------|---------------
1 | Default Organization | Default_Organization |
12 | org3 | org3-label | this is an org
---|----------------------|----------------------|---------------