Configurable timeouts in volume migration tasks

Volume migration tasks involve moving large amounts of data between Source and
Destination clouds. Those tasks might be long-running. Operators might want to
increase timeouts for this class of tasks so they don't fail erroneously.
Configuration file includes PARAMETERS section that includes
volume_tasks_timeout parameter.

Example:

PARAMETERS:
    volume_tasks_timeout: 300

Conflicts:

	doc/samples/api-config.yaml