CP-8629: Direct PPA install

A build system behind a firewall cannot use apt-add-repository directly.
To workaround the problem, including the public key and adding the
repository directly as part of
install_prerequisites_for_python_unittests.sh script.

Signed-off-by: Mate Lakat <mate.lakat@citrix.com>
CP-8629: Install to temporary directory

While making snapwatchd, the binaries are also installed. If DESTDIR is
not set, it means that the binaries will be installed to the /
directory. Create a temporary directory, and set that as DESTDIR to
avoid polluting the root directory.

Signed-off-by: Mate Lakat <mate.lakat@citrix.com>
CP-8629: Add coverage and pylint

Install coverage and pylint modules to the virtual environment to be
able to retrieve metrics.

Signed-off-by: Mate Lakat <mate.lakat@citrix.com>
CP-8629: Run unittests with coverage

To measure unit test coverage.

Signed-off-by: Mate Lakat <mate.lakat@citrix.com>