Make sure @omero_host@ is never present (fix #12555).

This fixes the issue when an insecure connection is
requested and the connection string still contains
@omero_host@.

To test:
- verify that scripts still work,
- check that a sample script from the ticket description (https://trac.openmicroscopy.org.uk/ome/ticket/12555) works. Save the code to a file, send it to the server as a script and execute (`bin/omero script launch <id>`). No errors should be present.

/cc @joshmoore, @sbesson 

--no-rebase
For there to be multiple endpoints, someone would have had to configure the server and likely also the clients to accept the `Ice.Default.Router` string to be an array of endpoints in which case it is certainly valid to chose any one of them. In practice, no one I know of is currently doing that. There are a few locations which we would need to update to make OMERO completely flexible for use of `Ice.Default.Router`, including allowing pasting in some arbitrary string in the GUIs, so I don't see this being needed any time soon. /cc @jburel