Add serverl_url attribute to Launcher class
Add serverl_url attribute to Launcher class

This attribute holds the URL of WMT API server that submitted the
job. The URL is passed to the `--server-url` keyword of the slave
command. With this attribute, we no longer need to specify the server
URL in the `_extra_args` class attribute of QsubLauncher and
BashLauncher.

csdms/wmt-exe,csdms/wmt-exe,csdms/wmt-exe,csdms/wmt-exe