common: allow configuring tcp congestion control

Also set it to BBR on soyuz because it seems to greatly increase my
download speeds from mirror.pkgbuild.com; example download results:

   CUBIC: 60.75M in 28s (2.18 MB/s)
     BBR: 60.75M in 12s (5.12 MB/s)

BBR gets to my line's max rate very fast while CUBIC struggles and only
very slowly scales up. Not sure if my ISP is to blame but seeing as BBR
works so well, we might as well use it.

host_vars/soyuz.archlinux.org

@@ -6,5 +6,5 @@ ipv6_address: "2a01:4f8:173:1654::1"
 ipv6_netmask: "/128"
 ipv4_gateway: "138.201.206.65"
 ipv6_gateway: "fe80::1"
+tcp_congestion_control: "bbr"
 filesystem: btrfs
-

roles/common/tasks/main.yml

@@ -35,6 +35,13 @@
   notify:
     - restart networkd
 
+- name: configure tcp congestion control algorithm
+  sysctl:
+    name: net.ipv4.tcp_congestion_control
+    value: "{{tcp_congestion_control}}"
+    sysctl_file: /etc/sysctl.d/local.conf
+  when: tcp_congestion_control is defined
+
 - name: configure journald
   template: src={{item}}.j2 dest=/etc/systemd/{{item}} owner=root group=root mode=644
   with_items: