cylc test-battery: fix get-global-config for site.rc hosts.

This fixes the `get-global-config` test when more than one
`site.rc` host is specified. This is due to the `use login
shell` line appearing once per host, not just once.

The `sed` change takes the first occurrence of `use login
shell` following a line that looks like `[[localhost]]`.

A site.rc file that contains the following text:
```
[hosts]
    [[some_network_machine]]
        run directory = $HOME/cylc-run
        work directory = $HOME/cylc-run
        task communication method = pyro
        remote shell template = ssh -oBatchMode=yes %s
        use login shell = True
```

will cause this test to fail in current master, but will be
OK for this branch.

@hjoliver, please review. Is this what #795 refers to? (`cylc-get-global-config` vs `cylc-get-config`?)