Add support for proxy_requests property on sensu_check resources

[cwjohnston]

Expected Behavior

It should be possible to specify proxy_requests attributes directly on a sensu_check resource.

Current Behavior

proxy_requests attributes must be passed via sensu_check additional property.

Possible Solution

Add proxy_requests property to sensu_check resource, accepts Hash value.

Context

As of sensu version 0.28.0, we've added proxy check requests for generating permutations of a single check definition.

[xblitz]

would it make sens to have a resource to create "clients" to be consumed by the proxy_requests ?