locator::i_endpoint_snitch: align the _prefer_local parameter with _my_dc and _my_rack

Adjust the interface and distribution of prefer_local parameter read
from a snitch property file with the rest of similar parameters (e.g. dc and rack):
they are read and their values are distributed (copied) across all shards'
instances.

Signed-off-by: Vlad Zolotarov <vladz@cloudius-systems.com>

locator/gossiping_property_file_snitch.cc

@@ -214,6 +214,7 @@ future<> gossiping_property_file_snitch::reload_configuration() {
             if (engine().cpu_id() != _file_reader_cpu_id) {
                 local_s->set_my_dc(_my_dc);
                 local_s->set_my_rack(_my_rack);
+                local_s->set_prefer_local(_prefer_local);
             }
         }).then([this] {
             return seastar::async([this] {

locator/gossiping_property_file_snitch.hh

@@ -136,7 +136,6 @@ private:
     timer<> _file_reader;
     std::experimental::optional<timespec> _last_file_mod;
     std::istringstream _istrm;
-    bool _prefer_local = false;
     bool _file_reader_runs = false;
     unsigned _file_reader_cpu_id;
     shared_ptr<reconnectable_snitch_helper> _reconnectable_helper;

locator/production_snitch_base.hh

@@ -163,6 +163,10 @@ private:
         _my_rack = new_rack;
     }
 
+    virtual void set_prefer_local(bool prefer_local) override {
+        _prefer_local = prefer_local;
+    }
+
     void parse_property_file();
 
 protected:

locator/snitch_base.hh

@@ -145,6 +145,7 @@ public:
     // noop by default
     virtual void set_my_dc(const sstring& new_dc) {};
     virtual void set_my_rack(const sstring& new_rack) {};
+    virtual void set_prefer_local(bool prefer_local) {};
     virtual void set_local_private_addr(const sstring& addr_str) {};
 
     static distributed<snitch_ptr>& snitch_instance() {
@@ -429,6 +430,7 @@ private:
 protected:
     sstring _my_dc;
     sstring _my_rack;
+    bool _prefer_local = false;
 };
 
 } // namespace locator