Support user@domain Nextcloud usernames for XMPP Cloud Auth

[MarcelWaldvogel]

When an installation has Nextcloud account names of the form user@domain, this is not supported externalApi.php.

This fixes jsxc/xmpp-cloud-auth#13 by receiving the new domain parameter and checking for an account of the form user@domain first.

[MarcelWaldvogel]

The interesting part of the Travis log seems to be:

+./occ maintenance:install --database-name oc_autotest --database-user oc_autotest --database-pass --admin-user admin --admin-pass admin --database mysql
ownCloud is not installed - only a limited number of commands are available
ownCloud was successfully installed
+./occ app:enable ojsxc
ojsxc not found
+./occ config:import apps/ojsxc/tests/travis/config-redis.json
The file must contain a valid json array
+cd apps/ojsxc
./tests/travis/install-oc-master.sh: line 20: cd: apps/ojsxc: No such file or directory
The command "./tests/travis/install-$REPO-$BRANCH.sh" failed and exited with 1 during .
Your build has been stopped.

I'm not a Travis expert, but this seems to be a Travis/jsxc.nextcloud related problem, not due to my PR. Who can fix this?

[LEDfan]

@MarcelWaldvogel I'll fix this when my exams are over 😃 (it's unrelated to this PR)