provider/bootstrap: verify credentials early, provide user-friendly error message

Verify credentials early in the bootstrap process, and if authentication
fails, provide a user friendly error message. We also remove a redundant
error message from the bootstrap failure case.

This branch only addresses OpenStack; other providers will be addressed
separately.

Partially fixes https://bugs.launchpad.net/juju-core/+bug/1362072
provider/openstack: only verify credentials in Prepare, not Open
Merge pull request #864 from axw/lp1362072-providers-verify-credentials

provider/openstack: verify credentials early, provide user-friendly error message

Verify credentials early in the bootstrap process, and if authentication
fails, provide a user friendly error message. We also remove a redundant
error message from the bootstrap failure case.

This branch only addresses OpenStack; other providers will be addressed
separately.

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