Include hook path where supervisor searchs for conf files.

This allow the user to mount a path into /etc/supervisor/conf.d/hook.d and set
new programs or configurations, without the need to create explicitly a new
image with a conf file in the /etc/supervisor/conf.d/program.d path.
Change `*.conf` filenames.

This should reduce the chances of the user accidentally overwrite the
configuration for the given service, in case it chooses to extend the image
with more than one service.
Add memsql image.
Add image to support leveldb.