Add daemon manager with basic instance task creation

[fierlion]

Summary

This commit adds the daemonmanager package which will be used by the Agent to create internal managed daemon tasks/containers. It also expands the managed daemon definition and setup with additional functions to create and validate the daemons.

Implementation details

Managed Daemons now have setup validation requirements in their required mount points and healthcheck. Once a daemon is successfully created, it should be registered with its own daemon manager object. This object exposes methods to create the internal managed daemon task.

Still to come are the daemon configuration and tar files (see inline TODOs in ManagedDaemon.ImportAll()) and also integration with the agent startup and attachment/task payload handlers to create and add the daemons to the Agent's managed task set.

Note that much of this is borrowing heavily from the service connect manager: https://github.com/aws/amazon-ecs-agent/blob/master/agent/engine/serviceconnect/manager_linux.go#L299

Testing

Added new unit tests for all functions.

New tests cover the changes: Yes

Description for the changelog

Add daemon manager package with initial daemon task creation methods.

Licensing

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.