Change dhparam positional args to be inherited vars, standardize naming

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

generate-dhparam.sh

@@ -1,8 +1,9 @@
 #!/bin/bash -e
 
-# The first argument is the bit depth of the dhparam, or 4096 if unspecified
-DHPARAM_BITS=${1:-4096}
-GENERATE_DHPARAM=${2:-true}
+# DHPARAM_BITS is the bit depth of the dhparam, or 4096 if unspecified
+DHPARAM_BITS=${DHPARAM_BITS:-4096}
+# DHPARAM_GENERATION=false skips dhparam generation
+DHPARAM_GENERATION=${DHPARAM_GENERATION:-true}
 
 # If a dhparam file is not available, use the pre-generated one and generate a new one in the background.
 # Note that /etc/nginx/dhparam is a volume, so this dhparam will persist restarts.
@@ -26,7 +27,7 @@ if [[ -f $DHPARAM_FILE ]]; then
     fi
 fi
 
-if [[ $GENERATE_DHPARAM =~ ^[Ff][Aa][Ll][Ss][Ee]$ ]]; then
+if [[ $DHPARAM_GENERATION =~ ^[Ff][Aa][Ll][Ss][Ee]$ ]]; then
     echo "Skipping Diffie-Hellman parameters generation and Ignoring pre-generated dhparam.pem"
     exit 0
 fi