High: pengine: per-node control over resource discovery

By using the 'resource-discovery-enabled' node attribute, users
can disable/enable resource discovery (probes) on a per node
basis. By default, resource discovery is enabled still. No default
behavior has changed with the introduction of this option.
High: crm_resource: avoid deletion of lrm cache on node with resource discovery disabled.

This patch prevents users from deleting lrm cache on nodes that
have resource discovery disabled. Deleting the lrm cache in this
situation would result a failure to re-detect the resources currently
running on the node. Instead, lrm delete is skipped, but failcounts
are still cleaned up properly.