Allow to install public repository signing keys

[Sebastian Heinlein]

It should be possible to add a key from string or file which was used for repository signing. Furthermore removing should also be possible.

Here is a short implementation plan:

1. The role enums are defined in the enums module. There is already a
enum for installing keys, but it has a bad name
(ROLE_INSTALL_SIGNATURE). You could rename it to
ROLE_INSTALL_REPO_KEY_FILE.

2. Take the InstallFile method from the AptDaemon class as an example
for a new InstallRepoKeyFile method. As a starting point write a dummy
method that doesn't install anything but only prints the file path.

2. Add an install_repo_key_file method to the AptClient which calls the
InstallRepoKeyFile method of the daemon

3. Add an --install-repo-key option to the command line tool

4. Now you can test if your transaction works: Does the daemon prints
the value that you specified in the command line client?

5. Use the add_key method of softwareproperties.AptAuth method in the
InstallRepoKeyFile method to actually do something useful

6. Add tests to the InstallRepoKeyFile method: is this a valid file
path? can we read the file? Is it of the correct mime type?

FUTURE: Provide a RemoveRepoKey and a InstallRepoKey method. The later
one would accept a key as a string and not stored in a file.

[Olof Kindgren (flamingolof)]

This should be fixed by now