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.
cardiff: Removing confusing message

When no disk performance information was part of the results we had the
following message printed on the output :
 No mode found in this benchmark, exiting

This could have been read as an error while it isn't.

So let's remove this confusing message.
cardiff: Deleting output dir if exists in simulatenous plot mode

When plotting simulatenous traces, the output dir name is made of the
benchmark definition. When running it multiple times, cardiff was
failing because the output directory was already existing.

In such case, let's delete it as it only contains unexpected results.
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.
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.