Add option to import untrusted repo signing key

[digitaltom]

Similar to the feature #114 which allows to interactively accept a new repository EULA, it would be useful to be able to interactively (or via cms switch, config) import untrusted repo signing keys.

This for example affects the IBM extensions on SLE12, and the nvidia repos on SLE15, which are signed with a non SUSE signing key.

[hramrach]

There is still the question of how to verify the key, and how it is done when the repository is added in the installer where this problem does not exist.

For security reasons the key should not be untrusted but rather imported from a known key database.