allow mysql_* modules to get socket connection information from my.cnf

This treats socket just like the other connection information (user and passwd) and gets them from my.cnf if it can, when it's not explicitly passed as an option.

This is based on an earlier conversation on the ansible list: https://groups.google.com/forum/#!searchin/ansible-project/%22Michael$20Peters%22|sort:date/ansible-project/npSwZM_2OFU/kV5vgNMxqdMJ