Add rpc.dryad.ClientManager interface

Add rpc.dryad.ClientManager interface

This interface allows adding an abstraction layer between Dryad's
client implementation and usage. It can be used also for using
mocked up implementation od Dryad's client for test purposes.

The mock implementation of interface is provided in workers package.
It is generated using mockgen command:

mockgen	-package workers \
	-destination=workers/dryadclientmanager_mock_test.go \
	-write_package_comment=false \
	-mock_names ClientManager=MockDryadClientManager \
	git.tizen.org/tools/boruta/rpc/dryad ClientManager

Change-Id: I7366a0e68a355c98b898fb00ec8efb3c642a725d
Signed-off-by: Lukasz Wojciechowski <l.wojciechow@partner.samsung.com>