Align metricsDoc.EnvUUID serialization.

Follow the multi-environment document convention of serializing EnvUUID
as "env-uuid".
Fix 1373611 dont fail when backup is HA
Reinstate the 1.18 compatability for AddMachines.
Merge pull request #838 from howbazaar/fix-1.18-add-machine

Reinstate the 1.18 compatibility for AddMachines.

Fixes https://bugs.launchpad.net/juju-core/+bug/1373424
Merge pull request #836 from perrito666/fix_1373611

Fix 1373611 dont fail when backup is HA

Remove a verification on restore that is outdated.
In the past it was not possible to restore to an HA environment, 
this restriction has been lifted and this error is deprecated
Merge pull request #821 from cmars/metrics-env-uuid

Align metricsDoc.EnvUUID serialization.

Follow the multi-environment document convention of serializing EnvUUID
as "env-uuid".