Sub port interface high level design (#420)

* Initial sub port interface high level design

Signed-off-by: Wenda Ni <wenni@microsoft.com>

* Address comments

Signed-off-by: Wenda Ni <wenni@microsoft.com>

* Rename DOT1Q_INTERFACE table to SUB_INTERFACE table

Signed-off-by: Wenda Ni <wenni@microsoft.com>

* Make mtu not configurable in the current design

Signed-off-by: Wenda Ni <wenni@microsoft.com>

* Add more comments to using sub port as a bridge port, which is out of the scope of
this design

Signed-off-by: Wenda Ni <wenni@microsoft.com>

* Address comment

Signed-off-by: Wenda Ni <wenni@microsoft.com>

* Rename SUB_INTERFACE to VLAN_SUB_INTERFACE

Signed-off-by: Wenda Ni <wenni@microsoft.com>

* Clarify the use of iproute2

Signed-off-by: Wenda Ni <wenni@microsoft.com>

* Remove the need to create an object id for vlan

Signed-off-by: Wenda Ni <wenni@microsoft.com>

* Set admin status of a sub port interface

Signed-off-by: Wenda Ni <wenni@microsoft.com>

* Config ip address on a sub port interface using existing `config
interface ip` command

Signed-off-by: Wenda Ni <wenni@microsoft.com>

* Clarify the use of iproute2 package is internal to SONiC

Signed-off-by: Wenda Ni <wenni@microsoft.com>

* Add Appendix, difference between sub port interface and vlan interface

Signed-off-by: Wenda Ni <wenni@microsoft.com>

* Add miss policy as open questions

Signed-off-by: Wenda Ni <wenni@microsoft.com>

* List the attributes that can be changed at rum time in SAI spec

Signed-off-by: Wenda Ni <wenni@microsoft.com>

* No oper status in show command

Signed-off-by: Wenda Ni <wenni@microsoft.com>

* Update table of contents; add acknowledgment

Signed-off-by: Wenda Ni <wenni@microsoft.com>

* Add implementation reference

Signed-off-by: Wenda Ni <wenni@microsoft.com>

* Amend references

Signed-off-by: Wenda Ni <wenni@microsoft.com>

* Update sub port interface removal event flow

* Remove definite article

Signed-off-by: Wenda Ni <wenni@microsoft.com>

* Correct typo

Signed-off-by: Wenda Ni <wenni@microsoft.com>

* Update table of contents; update references

Signed-off-by: Wenda Ni <wenni@microsoft.com>

* Change wording

Signed-off-by: Wenda Ni <wenni@microsoft.com>

* Update usage examples

Signed-off-by: Wenda Ni <wenni@microsoft.com>

* Add CLI design for changing admin status

Signed-off-by: Wenda Ni <wenni@microsoft.com>

* Add an APPL_DB entry example for IPv6

Signed-off-by: Wenda Ni <wenni@microsoft.com>

* Add a STATE_DB entry example for IPv6

Signed-off-by: Wenda Ni <wenni@microsoft.com>

* Amend references

Signed-off-by: Wenda Ni <wenni@microsoft.com>

* Amend reference on add/del ip address CLI

Signed-off-by: Wenda Ni <wenni@microsoft.com>