Move exec_resource_agent from environment classes to LRM

Move exec_resource_agent from environment classes to LRM

With the changes and preparation work from the previous commits
we can now move the quite important method exec_resource_agent
from the Env classes to the LRM where it get's called.

The main advantage of this is that it now underlies regression
tests and that we do not have two separate methods where it
- does not make sense as agents them self should be virtualized
  not the method executing them
- adds more work as the must (or at least should) be in sync

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>