provision: added Restart method to the Provisioner interface

Related to #336.
testing: implement Restart in FakeProvisioner

Related to #336.
testing: clean restarts map on destroy

Related to #336.
testing: added Restarts method to FakeProvisioner

This method will return the number of restart calls to the given app.

Related to #336.
provision/juju: implement Restart method in the JujuProvisioner

Related to #336.
provision/local: implement Restart method in LocalProvisioner

This is just the same as JujuProvisioner.

Related to #336.
testing: cleaning restarts map in FakeProvisioner's Reset method
app: use Provisioner.Restart to restart apps

Closes #336.