Fix: Submit XML method update

[mamullen13316]

• minor fix to allow exclusion of the set operation in the XML payload of the submit_xml method. This is to solve a specific problem where the remove method doesn't work because the content to be removed is nested deeper than the first level. With this fix, the set_operation argument can be set to None so that the set operation is excluded from the XML payload. Then the payload of the object to be removed can be included, for example

payload = '''
<Remove>
       <AuthenticationServers>
           <RADIUSServer>
               <ServerName>{{ name }}</ServerName>
           </RADIUSServer>
       </AuthenticationServers>
   </Remove
'''

fw.submit_xml(template_data=payload, template_vars=template_vars, set_operation=None)