caddy-frontend: Dependencies on instances

Create caddyprofiledeps egg with dummy noop recipe.

Thanks to setting dependencies of this egg and enabling it on the instance
profile, buildout will install eggs during software run and activate them
during instance run.

No existing egg (like slapos.cookbook) is used, as this technique is to allow
profile/software release developer to choose required eggs used during
instantiation.

Another apporach would be to add dependency for validators in
slapos.recipe.template (in install_requires).