Avoid parsing of hostname from router string by setting _client on CLI.

* Importing an image file back into omero via import in omero.cli.CLI
requires setting a session UUID and specifying the hostname (not always
'localhost' since the OMERO.grid can be quite complex).
* It should be possible to simply pass a copy of the client to the CLI
object, and avoid such configuration. Unfortunately, there is a bug that
prevented the 'omero.host' property to be configured.
* This commit works around this bug by setting the property ourselves
(fix from Josh Moore -- see link to the mailing list on source).
* the previous solution created a process with the session UUID and
hostname on the name which made it very easy for anyone listening
to take control of the session. This solution also fixes it.
Add get_parent() method to pipe_block that defaults to download an ome.tiff.