Add a Monoid instance for FrozenSet
Add a Monoid instance for FrozenSet

.. so that it's easier to combine the values together.

Signed-off-by: Petr Pudlak <9d19625f8ee825a2c398462ed335cbcf98f79c32@google.com>
Reviewed-by: Helga Velroyen <2d0126b1ec6f9ec5cbec4a7012aa14faeaef10f5@google.com>

andir/ganeti,leshchevds/ganeti,ganeti/ganeti,yiannist/ganeti,mbakke/ganeti,bitemyapp/ganeti,ganeti/ganeti,leshchevds/ganeti,bitemyapp/ganeti,yiannist/ganeti,leshchevds/ganeti,andir/ganeti,onponomarev/ganeti,andir/ganeti,mbakke/ganeti,ganeti/ganeti,grnet/snf-ganeti,grnet/snf-ganeti,yiannist/ganeti,onponomarev/ganeti,mbakke/ganeti