Avoid `TypeError` when desired_capacity is not provided to `ec2_asg` module (#5501)

Moving the "check if min_size/max_size/desired_capacity..." code to execute BEFORE the desired_capacity code is used in the following operation:
num_new_inst_needed = desired_capacity - len(new_instances)

Otherwise the following exception occurs when desired_capacity is not specified and you're replacing instances:
    num_new_inst_needed = desired_capacity - len(new_instances)
TypeError: unsupported operand type(s) for -: 'NoneType' and 'int'

Stack Trace:

An exception occurred during task execution. The full traceback is:
Traceback (most recent call last):
  File "/var/lib/awx/.ansible/tmp/ansible-tmp-1478229985.74-62334493713074/ec2_asg", line 3044, in <module>
    main()
  File "/var/lib/awx/.ansible/tmp/ansible-tmp-1478229985.74-62334493713074/ec2_asg", line 3038, in main
    replace_changed, asg_properties=replace(connection, module)
  File "/var/lib/awx/.ansible/tmp/ansible-tmp-1478229985.74-62334493713074/ec2_asg", line 2778, in replace
    num_new_inst_needed = desired_capacity - len(new_instances)
TypeError: unsupported operand type(s) for -: 'NoneType' and 'int'

fatal: [localhost]: FAILED! => {"changed": false, "failed": true, "invocation": {"module_name": "ec2_asg"}, "module_stderr": "Traceback (most recent call last):\n  File \"/var/lib/awx/.ansible/tmp/ansible-tmp-1478229985.74-62334493713074/ec2_asg\", line 3044, in <module>\n    main()\n  File \"/var/lib/awx/.ansible/tmp/ansible-tmp-1478229985.74-62334493713074/ec2_asg\", line 3038, in main\n    replace_changed, asg_properties=replace(connection, module)\n  File \"/var/lib/awx/.ansible/tmp/ansible-tmp-1478229985.74-62334493713074/ec2_asg\", line 2778, in replace\n    num_new_inst_needed = desired_capacity - len(new_instances)\nTypeError: unsupported operand type(s) for -: 'NoneType' and 'int'\n", "module_stdout": "", "msg": "MODULE FAILURE", "parsed": false}
	to retry, use: --limit @

lib/ansible/modules/cloud/amazon/ec2_asg.py

@@ -611,6 +611,14 @@ def replace(connection, module):
     instances = props['instances']
     if replace_instances:
         instances = replace_instances
+        
+    #check if min_size/max_size/desired capacity have been specified and if not use ASG values
+    if min_size is None:
+        min_size = as_group.min_size
+    if max_size is None:
+        max_size = as_group.max_size
+    if desired_capacity is None:
+        desired_capacity = as_group.desired_capacity
     # check to see if instances are replaceable if checking launch configs
 
     new_instances, old_instances = get_instances_by_lc(props, lc_check, instances)
@@ -633,14 +641,7 @@ def replace(connection, module):
     if not old_instances:
         changed = False
         return(changed, props)
-
-    #check if min_size/max_size/desired capacity have been specified and if not use ASG values
-    if min_size is None:
-        min_size = as_group.min_size
-    if max_size is None:
-        max_size = as_group.max_size
-    if desired_capacity is None:
-        desired_capacity = as_group.desired_capacity
+      
     # set temporary settings and wait for them to be reached
     # This should get overwritten if the number of instances left is less than the batch size.