Fix DatabaseUpgradeChecker in standalone context

When running DatabaseUpgradeChecker as a standalone program _dao will not
be injected.  Still create an instance of VersionDaoImpl in constructor
and when DatabaseUpgradeChecker is ran in the mgmt server it will be
overwritten by the injected value.
diff --git a/engine/schema/src/com/cloud/upgrade/DatabaseUpgradeChecker.java b/engine/schema/src/com/cloud/upgrade/DatabaseUpgradeChecker.java
index 1afd056..6020bbb 100755
--- a/engine/schema/src/com/cloud/upgrade/DatabaseUpgradeChecker.java
+++ b/engine/schema/src/com/cloud/upgrade/DatabaseUpgradeChecker.java
@@ -67,6 +67,7 @@
 import com.cloud.upgrade.dao.UpgradeSnapshot217to224;
 import com.cloud.upgrade.dao.UpgradeSnapshot223to224;
 import com.cloud.upgrade.dao.VersionDao;
+import com.cloud.upgrade.dao.VersionDaoImpl;
 import com.cloud.upgrade.dao.VersionVO;
 import com.cloud.upgrade.dao.VersionVO.Step;
 import com.cloud.utils.component.SystemIntegrityChecker;
@@ -85,6 +86,8 @@
     VersionDao _dao;
 
     public DatabaseUpgradeChecker() {
+        _dao = new VersionDaoImpl();
+
         _upgradeMap.put("2.1.7", new DbUpgrade[] {new Upgrade217to218(), new Upgrade218to22(), new Upgrade221to222(),
                 new UpgradeSnapshot217to224(), new Upgrade222to224(), new Upgrade224to225(), new Upgrade225to226(),
                 new Upgrade227to228(), new Upgrade228to229(), new Upgrade229to2210(), new Upgrade2210to2211(),