aria2: Deploy aria2 download manager

The *aria2* role installs the *aria2* download manager and sets it up to
run as a system service with RPC enabled. It also sets up the web UI,
though that must be installed manually from an archive, for now.

roles/aria2/templates/aria2.conf.j2

@@ -0,0 +1,17 @@
+dir={{ aria2_download_dir }}
+bt-exclude-tracker=*
+bt-prioritize-piece=head,tail
+bt-require-crypto=true
+dht-listen-port={{ aria2_dht_listen_ports }}
+dht-entry-point=dht.transmissionbt.com:6881
+dht-file-path=/var/lib/aria2/dht.dat
+dht-file-path6=/var/lib/aria2/dht6.dat
+listen-port={{ aria2_listen_ports }}
+enable-rpc=true
+rpc-listen-all=true
+log=/var/log/aria2.log
+log-level=notice
+continue
+save-session=/var/lib/aria2/aria2.state
+input-file=/var/lib/aria2/aria2.state
+save-session-interval=10