MINOR: reg-tests: add a way to add service dependency

MINOR: reg-tests: add a way to add service dependency

I was looking at writing a simple first test for prometheus but I
realised there is no proper way to exclude it if haproxy was not built
with prometheus plugin.

Today we have `REQUIRE_OPTIONS` in reg-tests which is based on `Feature
list` from `haproxy -vv`. Those options are coming from the Makefile
itself.

A plugin is build this way:
  EXTRA_OBJS="contrib/prometheus-exporter/service-prometheus.o"

It does register service actions through `service_keywords_register`.
Those are listed through `list_services` in `haproxy -vv`.
To facilitate parsing, I slightly changed the output to a single line
and integrate it in regtests shell script so that we can now specify a
dependency while writing a reg-test for prometheus, e.g:

  #REQUIRE_SERVICE=prometheus-exporter
  #REQUIRE_SERVICES=prometheus-exporter,foo

There might be other ways to handle this, but that's the cleanest I
found; I understand people might be concerned by this output change in
`haproxy -vv` which goes from:

  Available services :
          foo
          bar

to:

  Available services : foo bar

Signed-off-by: William Dauchy <wdauchy@gmail.com>