PR #913 added `DHPARAM_GENERATION` as a positional argument to
generate-dhparam.sh. However, since it was the second positional argument,
`DHPARAM_BITS` would also have to be defined or `DHPARAM_GENERATION` would be
read into `DHPARAM_BITS`. This changes the arguments to be inherited variables
which do not depend on order, just declaration.
Also change instances of `GENERATE_DHPARAM` to `DHPARAM_GENERATION` since it's
unnecessary to have another variable. I think `GENERATE_DHPARAM` is actually a
better name (verb vs. noun), but `DHPARAM_GENERATION` is already defined and
may break someone if changed.
Addresses https://github.com/jwilder/nginx-proxy/pull/913#issuecomment-476014691
