caddy-frontend: Set open files soft limit

Caddy by itself does not raise soft limit of open files, so it has to be set
by the wrapper.

As slapos.cookbook:wrapper can't be used for such case, the
slapos.recipe.template:jinja2 recipe inline style is used to have full
control over the created wrapper.

/reviewed-on !678
1 job for glib in 0 seconds
Status Job ID Name Coverage
  External
failed SlapOS.SoftwareReleases.IntegrationTest-Master

04:00:24