Added command line support for Active Slave sync

I wanted to setup master/slave nis server, but there was no command line option for me to enable the "Active Slave NIS server exists". 
If we don't enable this the data is not pushed from master to slave server, and same has said in the docs.

So this patch works great. hopping to see in next update.
But this is not about the default value. The check box is checked for ``!nopush`` value, not for ``nopush``. You can also see at the line https://github.com/ashish1099/yast-nis-server/blob/master/src/include/nis_server/master.rb#L242 that the checkbox reflects opposite value