use as default vm size and clear the test cases

Author: weiliu2

src/azure-cli/azure/cli/command_modules/acs/_consts.py

@@ -43,8 +43,8 @@
 CONST_AVAILABILITY_SET = "AvailabilitySet"
 
 # vm size
-CONST_DEFAULT_NODE_VM_SIZE = "Standard_DS2_v2"
-CONST_DEFAULT_WINDOWS_NODE_VM_SIZE = "Standard_D2s_v3"
+CONST_DEFAULT_NODE_VM_SIZE = "" 
+CONST_DEFAULT_WINDOWS_NODE_VM_SIZE = ""
 
 # gpu instance
 CONST_GPU_INSTANCE_PROFILE_MIG1_G = "MIG1g"