subnets: store ProviderNetworkId in state and display it

[babbageclunk]

Description of change

The GCE provider reports the id of the network for subnets that it finds. This information is potentially useful (for example to distinguish between subnets with overlapping ranges that are actually on different networks), so we should keep track of it in the database and display it.

Includes a driveby fix to remove the embedded interfaces from the shims in apiserver/networkingcommon - they just masked possible errors with unimplemented interface methods.

QA steps

Bootstrap a controller on GCE against a project with subnets (it needs to have more than just a legacy network). Run juju subnets - the name of the network should be reported (as provider-network-id) against each subnet.

[babbageclunk]

!!build!!

[wallyworld]

A quibble about the naming consistency

[wallyworld]

SubnetProviderId vs ProviderNetworkId

Ideally these would be consistent.
ie
SubnetProviderId
NetworkProviderId

I prefer the other way around but what's done is done

[babbageclunk]

(as discussed on IRC)
Unfortunately these names are already inconsistent in the codebase. I picked this version based on the existing ones in github.com/juju/juju/network/network.go:
On InterfaceInfo

• ProviderSubnetId
• ProviderSpaceId
• ProviderVLANId
• ProviderAddressId

On SubnetInfo:

• SpaceProviderId

I could name it NetworkProviderId on network.SubnetInfo and params.AddSubnetArgs (and ProviderNetworkId elsewhere) so it's consistent within a type, but that seems more confusing (and means more tech debt work if/when we decide to make them all consistent).

[babbageclunk]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju

[babbageclunk]

!!chittychitty!!

[jujubot]

Build failed: Tests failed
build url: http://juju-ci.vapour.ws:8080/job/github-merge-juju/10553

[babbageclunk]

Need to update the dependency on description once juju/description#7 is merged.

[babbageclunk]

Added the needed migration bits.

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju