Rename UserConfig.DeploymentFile to DeploymentManifestPath

- This is not reverse compatible with old deployment.json files. The path to
  the deployment manifest will be forgotten when you upgrade.

Signed-off-by: Karl Isenberg <kisenberg@pivotal.io>
CPI.create_vm should take a unique AgentID, not a DeploymentID

- All CPI calls take a DirectorID in their context, not DeploymentID
- Move http agent client into agentclient/http package
- Reordered Cloud.CreateStemcell args to match CPI.create_stemcell

[#82914700]

Signed-off-by: Karl Isenberg <kisenberg@pivotal.io>
Added lazy loaded DirectorID & DeploymentID

- DirectorID is used by the agent client (so the agent knows who to talk back
  to)
- DeploymentID is used by the cloud to pass in the context of each CPI call

[#82914700]

Signed-off-by: Tyler Schultz <tschultz@pivotal.io>