[cli-ref] Add support for multiple clients
[cli-ref] Add support for multiple clients

- added ability to supply multiple clients at a time.
- added flag to continue processing clients even if an error
  occurs.
- fixed issue where an empty file was created if the client
  wasn't found.
- fixed issue where specifying --all-api and --all-manage would
  result in only --all-api being generated.

Change-Id: I8586279d884e1223ee7eb8ab12f50aa5ddc82561

openstack/openstack-doc-tools,openstack/openstack-doc-tools