Fix the issue with defining FTP log depot.

The method connect_ftp has been deleted when some of the logic have been moved from LogFile into FileDepot and its subclasses. 
Consequently the FTP log depot validation failed.

https://bugzilla.redhat.com/show_bug.cgi?id=1151173
I don't like calling a subclass in the parent class like this.  It violates basic OO inheritance.