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]].
Merge pull request #852 from benfitzpatrick/cylc-test-battery.fix-hosts-get-global-config

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