Fix output of unmet dependences, add support for 'udeb' packages.

- When debian package manager selected to install older version
  of a package, but newer version requested by other package,
  then output with error is slightly different then was expected:
  Depends: package-name (= v2.0) but (v1.0) is to be installed

- Add support for 'udeb' packages for debian-installer.
  Now *.udeb packages can be placed to the folder
  /var/www/nailgun/ubuntu/x86_64/pool/debian-installer , repository
  can be updated with the sctipt 'regenerate_ubuntu_repo' with the
  same way as for *.deb packages.
  Also 'package_testing_deb' now can be used for testing gerrit
  reviews that contain any of *.deb or *.udeb packages.

Change-Id: Ib11546f7aa75e23e7f879170416c7183ff519df7
Closes-Bug: #1391569
Merge "Fix output of unmet dependences, add support for 'udeb' packages."