implemented DBClientHatohol::getNumberOfGoodHosts().

Note: testDBClientHatohol::test_getNumberOfGoodHosts failed.
But the expected value '1' may be wrong. I'll fix the test.

===============================================================================
Failure: testDBClientHatohol::test_getNumberOfGoodHosts
<expected == actual>
expected: <1>
  actual: <0>
./testDBClientHatohol.cc:210: void testDBClientHatohol::_assertGetNumberOfHostsWithStatus(bool): cppcut_assert_equal(expected, actual, )
./testDBClientHatohol.cc:389: void testDBClientHatohol::test_getNumberOfGoodHosts(): _assertGetNumberOfHostsWithStatus(true)