[RFC] Follow idea of immutable /usr vs. mutable overrides in /etc

There are many practical reasons why we want to copy this growingly
popular scheme, while enabling users to modify the agents per their
needs, for instance:

- having solely static data in `/usr` allows one to share that as
read-only (or sparsely utilized copy-on-write) mount point with
their VMs and containers so as to save space

- no conflict-on-update issue

Hence my expectation is that OCF standard will address this,
presumably in `resource-agent-api.md` by replacing

> The Resource Agents are located in subdirectories under
> `/usr/ocf/resource.d`.

with something like

> The Resource Agents are located in subdirectories under
> `/usr/ocf/resource.d`. OCF X.Y compliant RM shall first consult
> `/etc/ocf/resource.d` path for existence of the requested agent,
> which, when present, takes a precedence in the agent lookup.
> This makes for convenient customization of existing agents without
> altering them at the stated standard location, and in turn,
> simplifying a revert to stock configuration, coexistence with
> package updates, and possibly locked-down use of `/usr` mount
> point. The agent lookup based on the file presence is definite,
> any further issue, like file not being executable, notwithstanding.