Remove role dependencies of sensu-client.

Having several roles depend on sensu-client was causing sensu-client
role to be applied multiple times, lengthening deployments.

The only real dependency was for the "restart sensu client" handler.

This change introduces a new role, common-handlers, which contains
the "restart sensu client" handler.

Other roles can now depend on this common-handlers role, without causing
any tasks to be run repeatedly.
Merge pull request #92 from blueboxgroup/dont-repeat-sensu

Remove role dependencies of sensu-client.