This patch implements the handling of SSH keys, when a new
node is added. It introduces the new RPC call 'ssh_add_key',
which is called to the master's noded when a new node is
added. In the backend implementation, noded takes care of
distributing the new node's SSH key information to all
other nodes in the cluster which are supposed to have
this information.

Note: It was rather tedious to test the backend function,
because it was calling many other functions which would
have needed to be mocked. Instead I added the public key
file as a parameter, because this way I could at least
reduce the complexity of the test setup and at the same
time have direct access to the file that gets manipulated.

Also Note: Up till now, there is still only the common
cluster SSH key around. I wanted to have some
infrastructure in place, before actually individual keys
are generated.

Signed-off-by: Helga Velroyen <helgav@google.com>
Reviewed-by: Petr Pudlak <pudlak@google.com>