add Huawei device support through YANG

[nickylba]

The existing huawei.py device having some private capabilities in , like "http://www.huawei.com/netconf/capability/exchange/1.0", which tell the connected NE (network element) to work in private work-mode.

In order to ask NE to work in standard way that accpeting only RFC compatible rpc and reply with standard rpc-reply, huaweiyang.py is added

[coveralls]

Coverage decreased (-0.6%) to 68.465% when pulling 4c95477 on nickylba:master into 23df9a7 on ncclient:master.

[coveralls]

Coverage decreased (-1.2%) to 68.044% when pulling ad132ec on nickylba:master into 434b03b on ncclient:master.

[einarnn]

Can you please explain in more detail what this PR is attempting to achieve?

[nickylba]

Hi, the Huawei routes can work in two mode:

1. schema: in this mode it can accept rpc in format like below, which have some private attribute. This is private behaviour of huawei routers

<rpc message-id="101" xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
  <get-config>
    <source>
      <candidate/>
    </source>
    <filter>
      <ifm xmlns="http://www.huawei.com/netconf/vrp" content-version="1.0" format-version="1.0">
      </ifm>
    </filter>
  </get-config>
</rpc>

2. yang: in this mode it can accept rpc in RFC format (must not have private attribute)

<rpc message-id="101" xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
  <get-config>
    <source>
      <candidate/>
    </source>
    <filter>
      <ifm xmlns="http://www.huawei.com/netconf/vrp/huawei-ifm">
      </ifm>
    </filter>
  </get-config>
</rpc>

The key difference in the rpc is the xmlns/format-version/content-version in the node "ifm". And We are using "hello" to to diff this two work-mode now, if it carry any private capabilities (ex. 'http://www.huawei.com/netconf/capability/exchange/1.0') presentd in huawei.py, it will work in schema-mode; if it carry only base 1.0 (urn:ietf:params:netconf:base:1.0) OR base 1.1, it will work in yang-mode

so we need another huaweiyang.py to connect to the huawei router in yang mode

thanks