CLOUDSTACK-4540: Set execute.in.sequence.network.element.commands=true by default

The things is, VR would take time to execute the commands, say it would need
time t1(which is greater than 0).

And the interval between parallel deployment is t2(which can be almost 0).  In
any case, VR need to handle commands in sequence internally, so if t1 > t2, then
the new task in the VR would wait longer and longer to execute, then some
commands result in timeout ultimately. No matter how long the timeout is, if
there are enough big number of queued task for VR, the last ones can timeout.

Currently VR has a robust mechanism to sequence the jobs internal and I
confirmed in this case, it works well. But there is no way to fix this issue if
VR is already 100% load at all time.

Probably we can improve the speed of VR internal executing, but seems the
ultimate answer is: set execute.in.sequence.network.element.commands to true. VR
doesn’t know how long it would take for mgmt. server to timeout, only mgmt.
server knows that.

By setting sequence execution in VR to "true", mgmt server would handle the
commands in a queue, make sure that VR won't be overload and report unexpected
failure just because it's overload.

server/src/com/cloud/configuration/Config.java

@@ -1878,9 +1878,9 @@ public enum Config {
             NetworkOrchestrationService.class,
             Boolean.class,
             "execute.in.sequence.network.element.commands",
-            "false",
+            "true",
             "If set to true, DhcpEntryCommand, SavePasswordCommand, UserDataCommand, VmDataCommand will be synchronized on the agent side."
-                + " If set to false, these commands become asynchronous. Default value is false.",
+                + " If set to false, these commands become asynchronous. Default value is true.",
             null),
 
     UCSSyncBladeInterval(