nixos: Provide a defaultText for type = package

We don't want to build all those things along with the manual, so that's
what the defaultText attribute is for.

Unfortunately a few of them were missing, so let's add them.

Signed-off-by: aszlig <[e-mail]>

nixos/modules/services/computing/slurm/slurm.nix

···
       package = mkOption {
         type = types.package;
         default = pkgs.slurm-llnl;
+        defaultText = "pkgs.slurm-llnl";
         example = literalExample "pkgs.slurm-llnl-full";
         description = ''
           The packge to use for slurm binaries.

nixos/modules/services/misc/matrix-synapse.nix

···
       package = mkOption {
         type = types.package;
         default = pkgs.matrix-synapse;
+        defaultText = "pkgs.matrix-synapse";
         description = ''
           Overridable attribute of the matrix synapse server package to use.
         '';

nixos/modules/services/misc/plex.nix

···
       package = mkOption {
         type = types.package;
         default = pkgs.plex;
+        defaultText = "pkgs.plex";
         description = ''
           The Plex package to use. Plex subscribers may wish to use their own
           package here, pointing to subscriber-only server versions.

nixos/modules/services/networking/consul.nix

···
       package = mkOption {
         type = types.package;
         default = pkgs.consul;
+        defaultText = "pkgs.consul";
         description = ''
           The package used for the Consul agent and CLI.
         '';

nixos/modules/services/networking/ejabberd.nix

···
       package = mkOption {
         type = types.package;
         default = pkgs.ejabberd;
+        defaultText = "pkgs.ejabberd";
         description = "ejabberd server package to use";
       };
 

nixos/modules/services/security/haka.nix

···
 
       package = mkOption {
         default = pkgs.haka;
+        defaultText = "pkgs.haka";
         type = types.package;
         description = "
           Which Haka derivation to use.

nixos/modules/services/web-servers/apache-httpd/owncloud.nix

···
     package = mkOption {
       type = types.package;
       default = pkgs.owncloud70;
+      defaultText = "pkgs.owncloud70";
       example = literalExample "pkgs.owncloud70";
       description = ''
           PostgreSQL package to use.