health: Do not copy removed netdetect.py script

netdetect.py script has been removed in 232acfb, copying it causes
the role to fail to build.
build/base: don't install selinux-policy-default on Debian

Currently, SElinux in eDeploy images is only supported for Red Hat
deployments.
While some work is done to bring full Debian support, don't install the
package.
This patch could be reverted later if needed to re-enable again the
package.
health-server: runtime is a generic parameter

Getting the runtime is a generic option of jobs, so let's use it in
parse_jobs() instead of getting it computed by all benchmarks tests.
doc: Adding {D}AHC documentation

This commit add the rst documentation for AHC & DAHC.

This includes the description of why benchmarking infrastructures are
important, what are the concepts of AHC and how to use it.