remove load_connection_file from consoleapp

The ConnectionFileMixin class should be able to handle this, keeping
things more DRY.
remove load_connection_file from kernelapp
again, just keeping things DRY-er
remove redundant parts of kernelapp

these live in the ConnectionFileMixin
more DRYing out of consoleapp
Merge pull request #5760 from ivanov/connection-mixin

DRY out load_connection_file implementations