Support subnet placement on Azure

[wallyworld]

Description of change

Azure supports subnet placement, ie --to subnet=foo
When subnet placement is used, the default hard coded subnets are no longer created.
Also as a driveby pass a params struct in a couple of places to avoid passing too many args to a function.

QA steps

Without placement, confirm that bootstrap and add-model and deploy works as before.

Set up a virtual network (test-vn) with subnets in resource group test-rg

juju add-model --config network=test-rg/test-vn
juju deploy mysql --to subnet=subnet-one

Check the NIC on the machine to confirm the subnet is used

Add a space

juju add-space foo 172.12.1.0/24

Deploy with a space constraint and placement

juju deploy mariadb --constraints="spaces=foo" --to subnet=subnet-one

[ycliuhw]

lgtm, thanks

[wallyworld]

$$merge$$

[wallyworld]

azure subnet placement for bootstrap