wakunode2 config. adding new 'topic' config parameter.

This new parameter can be repeated and we are starting to deprecate the
parameter 'topics', that expected to receive a space-separated list of
pubsubtopic to subscribe to.

apps/wakunode2/app.nim

@@ -547,7 +547,16 @@ proc setupProtocols(node: WakuNode, conf: WakuNodeConf,
     peerExchangeHandler = some(handlePeerExchange)
 
   if conf.relay:
-    let pubsubTopics = conf.topics.split(" ")
+
+    var pubsubTopics = @[""]
+    if conf.topicsDeprecated != "":
+      if conf.topics != @["/waku/2/default-waku/proto"]:
+        return err("Please don't specify 'topics' and 'topic' simultaneously. Only use the 'topic' parameter")
+
+      # This clause (if conf.topicsDeprecated != "") should disapear in >= v0.18.0
+      pubsubTopics = conf.topicsDeprecated.split(" ")
+    else:
+      pubsubTopics = conf.topics
     try:
       await mountRelay(node, pubsubTopics, peerExchangeHandler = peerExchangeHandler)
     except CatchableError:

apps/wakunode2/config.nim

@@ -42,7 +42,6 @@ type
       defaultValue: newSeq[ProtectedTopic](0)
       name: "protected-topic" .}: seq[ProtectedTopic]
 
-
     ## Log configuration
     logLevel* {.
       desc: "Sets the log level for process. Supported levels: TRACE, DEBUG, INFO, NOTICE, WARN, ERROR or FATAL",
@@ -54,7 +53,6 @@ type
       defaultValue: logging.LogFormat.TEXT,
       name: "log-format" .}: logging.LogFormat
 
-
     ## General node config
     agentString* {.
       defaultValue: "nwaku",
@@ -207,11 +205,16 @@ type
       defaultValue: false
       name: "keep-alive" }: bool
 
-    topics* {.
-      desc: "Default topics to subscribe to (space separated list)."
-      defaultValue: "/waku/2/default-waku/proto"
+    topicsDeprecated* {.
+      desc: "Default topics to subscribe to (space separated list). DEPRECATED: please use repeated --topic argument instead."
+      defaultValue: ""
       name: "topics" .}: string
 
+    topics* {.
+      desc: "Default topic to subscribe to. Argument may be repeated."
+      defaultValue: @["/waku/2/default-waku/proto"]
+      name: "topic" .}: seq[string]
+
     ## Store and message store config
 
     store* {.