provider/azure: verify credentials during Prepare

Verify credentials while preparing environment configuration
for Azure, and display a helpful error message on failure.

Fixes https://bugs.launchpad.net/juju-core/+bug/1362072
Fix copy/paste
Merge pull request #1073 from axw/lp1362072-azure-verify-credentials-1.21

provider/azure: verify credentials during Prepare (backport)

Verify credentials while preparing environment configuration
for Azure, and display a helpful error message on failure.

Fixes https://bugs.launchpad.net/juju-core/+bug/1362072