Support tags for manual cloud machines

[Hemanth Nakkina]

Currently tags is not supported as Constraints for Manual cloud [1]

This is a feature request to support
* adding tags when machines are added to Manual cloud
* using tags as constraints to deploy applications on manual cloud machines

Usecase:

Project sunbeam [2] uses juju manual provider and designates some of the machines to deploy openstack control plane and rest of machines to deploy data plane components. Sunbeam uses juju terraform provider to deploy applications and specific machine id's are provided as part of terraform plan to deploy applications on specific machines.
With support for tags, sunbeam need not keep a track of machine id and the tag by itself and just use tags constraint in terraform plan.

[1] https://juju.is/docs/olm/constraint#heading--manual
[2] https://github.com/openstack-snaps/snap-openstack