• builds on support for multiple switches in sonic-sairedis
• new vslib switch BCM81724 implements a virtual gearbox phy.
• support for launching second (BCM81724 is supported by its own syncd)
• simple refactoring of tests to support switches by part number, still working with
  sairedis to support multiple switch in tests so BCM81724 will be a separate pull request)
• changed example context_config.json to reflect renaming of phy REDIS tables (see sonic-swss-common
  commit 292b08a3a80b24b23663020b37e6260039a311c0)

Note that a future commit to sonic-buildimage will be required to trigger launch of physyncd
(launching is based on device config files which are currently not present in sonic-buildimage).

Testing done in multiple environments (broadcom fork and pure upstream). Example CLI output
based on changes pushed to sonic-utilities (commit a6c4456f6965b79bf9d02ff1962070a5eae6ea55)
running in VS switch supporting BCM81724:

root@sonic:/home/admin# show gearbox interfaces status
PHY Id Interface MAC Lanes MAC Lane Speed PHY Lanes PHY Lane Speed Line Lanes Line Lane Speed Oper Admin

   1    Ethernet0  25,26,27,28               10G      200,201               20G           206                40G      up       up
   1    Ethernet4  29,30,31,32               10G      202,203               20G           207                40G      up       up
   1    Ethernet8  33,34,35,36               10G      204,205               20G           208                40G      up       up

HLD is located at https://github.com/Azure/SONiC/blob/b817a12fd89520d3fd26bbc5897487928e7f6de7/doc/gearbox/gearbox_mgr_design.md

Signed-off-by: syd.logan@broadcom.com