Add explicit tree type for `policy`

[calind]

weighted policy

If a policy is of type weighted is should build only a weighted tree, ignoring policy member's region. When saving in admin a policy of type weighted we should validate:

1. all policy members are in the same region
2. there is at least one policy member with weight > 0

lbr policy

If a policy is of type lbr it should build a lbr tree, regardless of policy member's region. When saving in admin a policy of type lbr we should validate:

1. there are policy members in at least 2 regions
2. each reagion has at least one policy member with weight > 0