build system: no need for trivial images/* files

Until now, whenever we added a new module in apps.git, e.g., apps/tomcat,
we also had to add a file images/tomcat.py in osv.git, so that
"make image=tomcat" would work.

This patch makes it unnecessary to have a images/tomcat.py - when one does
"make image=tomcat" and images/tomcat.py is absent, we try to build an
image containing the single module tomcat and its dependencies (which
is defined as usual in apps/modules.json and apps/*/module.py).

Non-default images, which contain more than one unrelated modules (e.g., both
mgmt and tests) can still be defined in files in images/).

Reviewed-by: Tomasz Grabiec <tgrabiec@gmail.com>
Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>