Consider default in use

[yuekyang]

Normally, it's okay to use 'create' operation on a node when it does not exist in a configuration data tree. However, if there are default values in use, 'create' operation could be invalid. The PR fixes this scenario when generating Netconf edit-config.

Unit tests are added.

(pyats-2401) [yuekyang@yangtest-lnx tests]$ python -m unittest test_ncdiff
Error in Netconf reply when getting /netconf-state/schemas from YANG module 'ietf-netconf-monitoring':
<rpc-reply xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
  <rpc-error>
    <error-type>rpc</error-type>
    <error-tag>missing-attribute</error-tag>
    <error-severity>error</error-severity>
    <error-info>
      <bad-attribute>message-id</bad-attribute>
      <bad-element>rpc</bad-element>
    </error-info>
  </rpc-error>
</rpc-reply>

Error in Netconf reply when getting /modules-state/module from YANG module 'ietf-yang-library':
<rpc-reply xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
  <rpc-error>
    <error-type>rpc</error-type>
    <error-tag>missing-attribute</error-tag>
    <error-severity>error</error-severity>
    <error-info>
      <bad-attribute>message-id</bad-attribute>
      <bad-element>rpc</bad-element>
    </error-info>
  </rpc-error>
</rpc-reply>

........................................
----------------------------------------------------------------------
Ran 40 tests in 0.593s

OK
(pyats-2401) [yuekyang@yangtest-lnx tests]$ python -m unittest test_running_config
...........................................
----------------------------------------------------------------------
Ran 43 tests in 0.030s

OK
(pyats-2401) [yuekyang@yangtest-lnx tests]$