Fix how migration's cli register db connection opt

It's not good to duplicate the same oslo.config options in one project
due to the fact that it'll be hadled incorrectly by different config
sample generation tools. The better option is to use CONF.import_opt
that will import oslo/db/sqla/session that already registers db connection
option with good description and backward compatibility support.

Change-Id: I71c4c0442464f2b4e88fb172f5d5e23daa133834
Merge "Fix how migration's cli register db connection opt"