#### What I did

Fixes sonic-net#2007 

Most of the children of `/BGP_NEIGHBOR/*` except `admin_status` are create-only field i.e. they can only be created with the neighbor but cannot be modified later.

Validated each attribute is read-only by the following steps:
* Delete a neighbor
* Add the neighbor back without the attribute under test e.g. `holdtime`
* show running config for the neighbor
* show neighbor config using `show ip bgp neighbor <ip>`
* Add just the attribute under test e.g. `holdtime`
* show running config for the neighbor -- we can see the attribute is added
* show neighbor config using `show ip bgp neighbor <ip>` -- we can see the attribute change did not take effect

Example for `holdtime`:
```sh
admin@vlab-01:~$ sudo config apply-patch remove-bgp-neighbor.json -i '' 
.
.
.
Patch applied successfully.
admin@vlab-01:~$ sudo config apply-patch remove-bgp-neighbor.json -i ''
.
.
.
Error: can't remove a non-existent object '10.0.0.57'
admin@vlab-01:~$ sudo config apply-patch add-bgp-neighbor-without-holdtime.json -i ''
Patch Applier: Patch application starting.
Patch Applier: Patch: [{"op": "add", "path": "/BGP_NEIGHBOR/10.0.0.57", "value": {"admin_status": "up", "asn": "64600", "keepalive": "3", "local_addr": "10.0.0.56", "name": "ARISTA01T1", "nhopself": "0", "rrclient": "0"}}]
.
.
.
Patch applied successfully.
admin@vlab-01:~$ show runningconfiguration all | grep 10.0.0.57 -A8
        "10.0.0.57": {
            "admin_status": "up",
            "asn": "64600",
            "keepalive": "3",
            "local_addr": "10.0.0.56",
            "name": "ARISTA01T1",
            "nhopself": "0",
            "rrclient": "0"
        },
admin@vlab-01:~$ show ip bgp neighbors 10.0.0.57
.
.
. 
  Hold time is 180, keepalive interval is 3 seconds
.
.
. 
admin@vlab-01:~$ sudo config apply-patch add-holdtime.json -i ''
Patch Applier: Patch application starting.
Patch Applier: Patch: [{"op": "add", "path": "/BGP_NEIGHBOR/10.0.0.57/holdtime", "value": "10"}]
.
.
. 
Patch applied successfully.
admin@vlab-01:~$ show runningconfiguration all | grep 10.0.0.57 -A10
        "10.0.0.57": {
            "admin_status": "up",
            "asn": "64600",
            "holdtime": "10",
            "keepalive": "3",
            "local_addr": "10.0.0.56",
            "name": "ARISTA01T1",
            "nhopself": "0",
            "rrclient": "0"
        },
        "10.0.0.59": {
admin@vlab-01:~$ show ip bgp neighbors 10.0.0.57
BGP neighbor is 10.0.0.57, remote AS 64600, local AS 65100, external link
.
.
. 
  Hold time is 180, keepalive interval is 3 seconds
.
.
. 
admin@vlab-01:~$ 
```

Also added a validation to `create-only` fields to reject moves that add their parents without them, because we would have to delete their parents again later and add it back. There is no point.
Example assume we have 2 fields marked with create-only namely x,y and they are under c. 
The patch would be:
```
{"op":"add", "path":"/a/b/c", "value":{"x":"value_x", "y":"value_y"}}
```
The generated moves would be:
```
{"op":"add", "path":"/a/b/c", "value":{"x":"value_x"}}
{"op":"remove", "path":"/a/b/c"}
{"op":"add", "path":"/a/b/c", "value":{"x":"value_x", "y":"value_y"}}
```

There is no point of the first 2 moves, because the `y` is create only and it will require the object to be deleted again then added. 


#### How I did it
Marked the fields as create only

#### How to verify it
unit-test

#### Previous command output (if the output of a command-line utility has changed)

#### New command output (if the output of a command-line utility has changed)