state: revert field name change

state.address.NetworkScope was renamed to
Scope, with no explicit bson field name
specified. This is a schema change, and
broke upgrade from 1.18 to 1.19.

Fixes https://bugs.launchpad.net/juju-core/+bug/1334773
Merge pull request #183 from axw/lp1334773-state-address-networkscope

state: revert field name change

state.address.NetworkScope was renamed to
Scope, with no explicit bson field name
specified. This is a schema change, and
broke upgrade from 1.18 to 1.19.

Fixes https://bugs.launchpad.net/juju-core/+bug/1334773