Start of mock FTP server for mavlink ftp unit test

Lorenz: I'm just pushing this up here so you can see it and agree on direction. My plan is to modify MockUAS to support mock mavlink plugins. The pull is the starting point for a mock FTP file server which can then be used to write unit testing to for QGCUASFIleManager code. A combination of MockUAS and MockMavlinkFileServer will allow me to write unit tests for QGCUASFileManager. My plan is to complete the mock server, write a FileManager unit test and then complete the implementation of QGCUASFileManager. After that I can move over to the Pixhawk side and complete the FTP implementation there if still needed.