Add whitelisting mechanism

This commit implements the whitelisting mechanism for the installability
health-check script.

It whitelists packages using the following rules:

* If a package is uninstallable and whitelisted, then marks it as skipped.
* If a package is uninstallable and not whitelisted, it marks it as failure.

A message is added to each package entry in the junit.xml file with the
package status.

This commit also adds the initial list of whitelisted packages inside
the whitelists/ directory.

Signed-off-by: Luis Araujo <luis.araujo@collabora.co.uk>