apt-cdrom should succeed if any drive succeeds

If there are multiple CD-ROM drives, `apt-cdrom add` will abort with an
error if any of the drives do not contain a Debian CD which is against
the documentation we have saying "a CD-ROM" and also scripts do not
expect it this way.

This patch modifies apt-cdrom to return success if any of the drives
succeeded. If failures occur, apt-cdrom will still continue trying all
the drives and report the last failure (if none of them succeeded).

The 'ident' command was also changed to match the new 'add' behavior.

Closes: 728153
remove duplication in pkgCdrom::Add and ::Ident

Git-Dch: Ignore