GanetiLockManager, remove default values

The nodes and instances parameters to the constructor are mandatory
anyway, as a value of None will fail when creating the LockSet. Rather
than fixing this adding code lines, since we never used the default
value, let's remove them and require that the parameters are passed.

This also fixes the only places where we inited GanetiLockManager with
keyed parameters and without arguments.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>