Secure Key Management

This patchset implements the Secure Key Management feature
as described in Gerrit 1429, enabling support for native charms
deployed to machines provisioned by the Resource Orchestrator.

- Implement GetPublicKey, which will provide the public key to be
injected into new machines
- Support machine placement, to provision an existing machine for use
with juju
- Automatically create a SSH keypair to be used for provisioning
- Add method to check if a charm is deployed (CI)
- Update integration tests to use new ssh key workflow

Signed-off-by: Adam Israel <adam.israel@canonical.com>
Change-Id: Iacd2f02800484fd90945f9b9c1ac2d8951115a76