env: Re-work environment code

* Instead of storing just a server_url which was necessary to
  regenerate the OpenVPN configs, instead store an env file.
* Move all the env parsing to `ovpn_genconfig` so that it can be re-run
  from genconfig instead of from `ovpn_init`.
* Remove all the parsing and env defaults except for genconfig.

NOTE: This breaks the older config method, uesrs will need to re-run
genconfig with an arg[1] as the previous server_url, this will create
the necessary env file the rest of the tools expect.

Example recovery for legacy users:

    host$ docker run --rm -it kylemanna/openvpn bash -l
    container# ovpn_genconfig $(cat /etc/openvpn/server_url)

bin/ovpn_init

@@ -6,21 +6,10 @@
 
 set -ex
 
-server_url=$1
+# Generate the ovpn env file
+ovpn_genconfig "$1"
 
-# Server name is in the form "udp://vpn.example.com:1194"
-if [[ "$server_url" =~ ^((udp|tcp)://)?([0-9a-zA-Z\.]+)(:([0-9]+))?$ ]]; then
-    proto=${BASH_REMATCH[2]};
-    cn=${BASH_REMATCH[3]};
-    port=${BASH_REMATCH[5]};
-else
-    echo "Common name not specified"
-    exit 1
-fi
-
-# Apply defaults
-[ -z "$proto" ] && proto=1194
-[ -z "$port" ] && port=udp
+source "$OPENVPN/ovpn_env.sh"
 
 # Specify "nopass" as arg[2] to make the CA insecure
 nopass=$2
@@ -46,9 +35,5 @@ openvpn --genkey --secret $OPENVPN/pki/ta.key
 #    [ -n "$ptr" ] && cn=$ptr || cn=$ip4
 #fi
 
-echo "$server_url" > $OPENVPN/server_url
-
 # For a server key with a password, manually init; this is autopilot
-easyrsa build-server-full $cn nopass
-
-ovpn_genconfig "$cn"
+easyrsa build-server-full "$OVPN_CN" nopass