Added the InfluxDBNotification plugin including a NixOS test

This adds a `InfluxDBNotification` plugin which is configured as:

```
<influxdb>
  url = http://127.0.0.1:8086
  db = hydra
</influxdb>
```

which will write a notification for every finished job to the
configured database in InfluxDB looking like:

```
hydra_build_status,cached=false,job=job,jobset=default,project=sample,repo=default,result=success,status=success,system=x86_64-linux build_id="1",build_status=0i,closure_size=584i,duration=0i,main_build_id="1",queued=0i,size=168i 1564156212
```

tests/setup-notifications-jobset.pl

@@ -0,0 +1,56 @@
+use LWP::UserAgent;
+use JSON;
+
+my $ua = LWP::UserAgent->new;
+$ua->cookie_jar({});
+
+sub request_json {
+    my ($opts) = @_;
+    my $req = HTTP::Request->new;
+    $req->method($opts->{method} or "GET");
+    $req->uri("http://localhost:3000$opts->{uri}");
+    $req->header(Accept => "application/json");
+    $req->header(Referer => "http://localhost:3000/") if $opts->{method} eq "POST";
+    $req->content(encode_json($opts->{data})) if defined $opts->{data};
+    my $res = $ua->request($req);
+    print $res->as_string();
+    return $res;
+}
+
+my $result = request_json({
+  uri => "/login",
+  method => "POST",
+  data => {
+    username => "root",
+    password => "foobar"
+  }
+});
+
+$result = request_json({
+  uri => '/project/sample',
+  method => 'PUT',
+  data => {
+    displayname => "Sample",
+    enabled => "1",
+    visible => "1",
+  }
+});
+
+$result = request_json({
+  uri => '/jobset/sample/default',
+  method => 'PUT',
+  data => {
+    nixexprpath => "default.nix",
+    nixexprinput => "my-src",
+    inputs => {
+      "my-src" => {
+        type => "path",
+        value => "/run/jobset"
+      }
+    },
+    enabled => "1",
+    visible => "1",
+    checkinterval => "5",
+    keepnr => 1
+  }
+});