[REFACTOR] extracted interface from DTM class.

Extracted several methods that handle connects and disconnects to
IConnectionManager interface.

Change-Id: Ie2fa67b87afbcbbf2692aec67e8a8eb7e9620051
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1605
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>