Impl bulk ops for instance interop helper.

* cleanup InstanceInteropHelper class;
* add support for bulk ops using context managers.

For example:

with instance.remote as remote:
    remote.execute_command('hostname')
    remote.send_file_to('/etc/hosts', hosts_file)

Improves blueprint instance-interop-helper

Change-Id: I806c7adad787e669b6f62e5f02494b554ea53bde
Merge "Impl bulk ops for instance interop helper."