Replace bash-specific syntax in ejabberdctl

Use plain POSIX shell syntax to match `.yml` configuration file names.  This is also slightly more correct, as it matches `.yml` only at the _end_ of the file name.

The `ejabberdctl` script can now be executed with non-bash shells again, but I _didn't_ change the shebang line back to `#!/bin/sh` (in case you want to use bash-specific stuff in the future).