Add validate() base method
Add validate() base method

Now that all backends implement a validate that
can be called to check if the backend is still
useable we can add a base method to the base class
that defines exactly what this new method does.

Change-Id: Ica4571970bad2de16ce2071a645abf24c25c110b

varunarya10/taskflow,jimbobhickville/taskflow,junneyang/taskflow,openstack/taskflow,openstack/taskflow,pombredanne/taskflow-1,citrix-openstack-build/taskflow,varunarya10/taskflow,jimbobhickville/taskflow,citrix-openstack-build/taskflow,junneyang/taskflow,pombredanne/taskflow-1