Ensure add-apt-repository is available

On Ubuntu 12.04, an additional repo is added using
the add-apt-repository command. This is provided
by the python-software-properties package, which may
not be installed by default.

This patch adds a check to see if the command is available,
and installs the package if it is not.

Change-Id: If1189a7e66b9b4c8eb527f7e9e1e1a32ad8c6e7a
Closes-Bug: #1331337
Merge "Ensure add-apt-repository is available"