warnings during compilation

Reported by @razvan on #1891 (closed) (1.9.8 sanity checks)

minor:

In file included from json_config_parser.cc:14:
../../../src/bin/dhcp4/dhcp4_srv.h:386:10: warning: 'shutdown' overrides a member function but is not marked 'override' [-Winconsistent-missing-override]
    void shutdown();
         ^
../../../src/lib/process/daemon.h:74:18: note: overridden virtual function is here
    virtual void shutdown();
                 ^

In file included from json_config_parser.cc:17:
../../../src/bin/dhcp6/dhcp6_srv.h:203:10: warning: 'shutdown' overrides a member function but is not marked 'override' [-Winconsistent-missing-override]
    void shutdown();
         ^
../../../src/lib/process/daemon.h:74:18: note: overridden virtual function is here
    virtual void shutdown();
                 ^

In file included from d2_update_mgr.cc:9:
In file included from ../../../src/bin/d2/d2_update_mgr.h:16:
../../../src/bin/d2/d2_cfg_mgr.h:283:25: warning: 'getConfigSummary' overrides a member function but is not marked 'override' [-Winconsistent-missing-override]
    virtual std::string getConfigSummary(const uint32_t selection);
                        ^
../../../src/lib/process/d_cfg_mgr.h:165:25: note: overridden virtual function is here
    virtual std::string getConfigSummary(const uint32_t selection) = 0;
                        ^
In file included from d2_update_mgr.cc:9:
In file included from ../../../src/bin/d2/d2_update_mgr.h:16:
../../../src/bin/d2/d2_cfg_mgr.h:295:5: warning: 'parse' overrides a member function but is not marked 'override' [-Winconsistent-missing-override]
    parse(isc::data::ConstElementPtr config, bool check_only);
    ^
../../../src/lib/process/d_cfg_mgr.h:226:40: note: overridden virtual function is here
    virtual isc::data::ConstElementPtr parse(isc::data::ConstElementPtr config,
                                       ^
In file included from d2_update_mgr.cc:9:
In file included from ../../../src/bin/d2/d2_update_mgr.h:16:
../../../src/bin/d2/d2_cfg_mgr.h:303:18: warning: 'setCfgDefaults' overrides a member function but is not marked 'override' [-Winconsistent-missing-override]
    virtual void setCfgDefaults(isc::data::ElementPtr mutable_config);
                 ^
../../../src/lib/process/d_cfg_mgr.h:188:18: note: overridden virtual function is here
    virtual void setCfgDefaults(isc::data::ElementPtr mutable_config);
                 ^
In file included from d2_update_mgr.cc:9:
In file included from ../../../src/bin/d2/d2_update_mgr.h:16:
../../../src/bin/d2/d2_cfg_mgr.h:314:32: warning: 'createNewContext' overrides a member function but is not marked 'override' [-Winconsistent-missing-override]
    virtual process::ConfigPtr createNewContext();
                               ^
../../../src/lib/process/d_cfg_mgr.h:199:23: note: overridden virtual function is here
    virtual ConfigPtr createNewContext() = 0;
                      ^
4 warnings generated.