Enable switch to join kubernetes master transparently, if configured

[renukamanavalan]

When configured, hostcfgd watches and call join/reset connection to master, depending on whether IP address is non-empty/empty. If master is not available at that time point, the kube_join script, creates a cron job to retry every 10 minutes, until the server is reachable.

Introduced a new table in config-db:

    "KUBERNETES_MASTER": {
        "SERVER": {
            "IP": "10.10.10.10",
            "insecure": "false"
        }
    }

In Minigraph:
Under <MetadataDeclaration>/<Devices>/<DeviceMetadata>/<Name>...</Name>/<Properties>,
added new entities as below
    <a:DeviceProperty>
       <a:Name>KubernetesMaster</a:Name>
       <a:Reference i:nil="true"/>
       <a:Value>10.10.10.10</a:Value>
    </a:DeviceProperty>
    <a:DeviceProperty>
       <a:Name>KubernetesInsecure</a:Name>
       <a:Reference i:nil="true"/>
       <a:Value>true</a:Value>
    </a:DeviceProperty>

1. Updated minigraph.py to read & write this new data
2. Updated hostcfgd to watch "KUBERNETES_MASTER" and call join/reset
   connection to master depending on if IP address is configured or not
3. Added helper scripts kube_join.sh & kube_reset.sh for join/reset respectively

Code algorithm:

[qiluo-msft]

As comments

[lgtm-com]

This pull request introduces 1 alert when merging 46ce50f into 4adf243 - view on LGTM.com

new alerts:

• 1 for Unused argument in a formatting call

[renukamanavalan]

retest vs please.

[renukamanavalan]

retest vsimage please.

Unblocked, one issue remain

[renukamanavalan]

Writing kube commands as python package. A sonic-utilities PR #962 is under review. An updated new PR will be raised for build image. Hence closing this PR.