Applying package profile on Xenial doesn't work

[Impact]

 * Users trying to apply a landscape package profile with a recent version of python-apt on xenial will get a stuck activity and an incomplete sources.list.d entry.

 * The change adds a [trusted=yes] option to temporary local sources created by landscape. Global sources and apt options

[Test Case]

 * install landscape-client and register it.

 * create and associate a package profile.

 * the packages are installed and /var/log/landscape/package-changer.log has no errors

[Regression Potential]

 * This is a backport of fixes already released since yakkety. The change has been released for a long time.

 * Change is agnostic of whether the issue is present or not and should work no matter the apt configuration.

 * The affected code path is limited to profiles, but is also used a lot in unit tests fixtures, thus making it reasonably exercised.

[Other Info]

 * This change was not previously required on xenial, as it was not required. Acquire::AllowInsecureRepositories was set to be backward compatible and was accepted by python-apt. Changes in python-apt now looks for APT::Get::AllowUnauthenticated.

 * We fix landcape-client instead of raising the python-apt regression because the new behaviour is consistent with later releases, which we already handle. Also, it decouples landscape from requiring specific configuration.

[Original Description]
 * install landscape-server-quickstart and landscape-api
 * Create a package profile "minimum packages" and add basic packages (byobu, vim, ubuntu-cloud-keyring). Enable for all computers
 * create a xenial container or machine and install/configure landscape-client

Note that the apply package profile gets stuck and doesn't progress. On the container that was created, running apt update shows:

sudo apt update
Get:1 file:/var/lib/landscape/client/package/binaries ./ InRelease
Ign:1 file:/var/lib/landscape/client/package/binaries ./ InRelease
Get:2 file:/var/lib/landscape/client/package/binaries ./ Release
Err:2 file:/var/lib/landscape/client/package/binaries ./ Release
  File not found - /var/lib/landscape/client/package/binaries/./Release (2: No such file or directory)
Get:3 http://mirror/ubuntu xenial InRelease [247 kB]
Hit:4 http://mirror/ubuntu xenial-updates InRelease
Hit:5 http://mirror/ubuntu xenial-backports InRelease
Hit:6 http://mirror/ubuntu xenial-security InRelease
Reading package lists... Done
E: The repository 'file:/var/lib/landscape/client/package/binaries ./ Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.

apt-cache policy landscape-client
landscape-client:
  Installed: 16.03-0ubuntu2.16.04.7
  Candidate: 16.03-0ubuntu2.16.04.7
  Version table:
 *** 16.03-0ubuntu2.16.04.7 500
        500 http://mirror/ubuntu xenial-updates/main amd64 Packages
        100 /var/lib/dpkg/status
     16.03-0ubuntu2 500
        500 http://mirror/ubuntu xenial/main amd64 Packages

This can be easily worked around by:
 * Remove the file /etc/apt/sources.list.d/_landscape-internal-facade.list
 * add-apt-repository ppa:landscape/19.01
 * apt install landscape-client

apt-cache policy landscape-client
landscape-client:
  Installed: 18.03~1070~git.8e52e1d~ubuntu16.04.1
  Candidate: 18.03~1070~git.8e52e1d~ubuntu16.04.1
  Version table:
 *** 18.03~1070~git.8e52e1d~ubuntu16.04.1 500
        500 http://ppa.launchpad.net/landscape/19.01/ubuntu xenial/main amd64 Packages
        100 /var/lib/dpkg/status
     16.03-0ubuntu2.16.04.7 500
        500 http://mirror.glacier.dev/ubuntu xenial-updates/main amd64 Packages
     16.03-0ubuntu2 500
        500 http://mirror.glacier.dev/ubuntu xenial/main amd64 Packages