MULE-6880: FTP responseTimeout has no effect

_ Setting responseTimeout on the FTPClient in the connector. A better solution could be to set that value right in the factory, but that requires changes inthe API.
_ Adding FtpConnectionFactory#createFtpClient to let test subclasses to override the default behavior.
(cherry picked from commit ed4ad38)
(cherry picked from commit d045c654941bb0ce54793ba877ebcc4a1e7b2935)

Conflicts:
	transports/ftp/src/test/java/org/mule/transport/ftp/AbstractFtpServerTestCase.java