Use version 0.6.0-SNAPSHOT of yangtools everywhere in controller.

Use version 0.6.0-SNAPSHOT of yangtools everywhere in controller.

As of yangtools commit f5e9689b95d945c22c7a38870681cab899326de9, all projects
in yangtools repo have version 0.6.0-SNAPSHOT. This commit bumps all
yangtools dependencies to this version.

Change-Id: I4aab7c67ee7ce99618a8935cb6b6fecc5688aace
Signed-off-by: Tomas Olvecky <tolvecky@cisco.com>