Add support for vendor/os: broadcom_icos.

[alepodj]

Add support for vendor/os: broadcom_icos.

Some example hardware models that i have access to and use the OS above:

Accton AS4610-54P 
Accton AS5610-52X
Quanta LY2R
Quanta LB9
DNI AG3448P-R

• Ran tests and black against added code:

broadcom_icos_test_results.txt

[alepodj]

For context in case question arises(ignore otherwise):

These are relatively new devices.
https://www.broadcom.com/products/ethernet-connectivity/software/fastpath

Broadcom ICOS/FASTPATH is an off-the-shelf (Linux based) network operating system (NOS), providing traditional L2 and L3 functions and management, with an API-structure for value-added applications and integration with provisioning and orchestration systems.

ICOS is a robust, programmable full featured and most widely deployed networking operating system for enterprise and data center networking. Its built-in arbitration between conventional and SDN flows allowing customers seamless integration to Next-Gen Open programmable networks.

FASTPATH production-ready networking software helps manufacturers achieve record-setting time-to-market performance for introducing new Ethernet products. In part, this advantage comes from the extensive feature set and integration capabilities that give Broadcom’s software unprecedented application flexibility.

Im currently working with them joined to Cisco routers on enterprise networks, also didnt see any apparent templates on ntc_templates for them so im also helping on that front by working with the fellas at ntc_templates to add a bunch of templates for these as well:

networktocode/ntc-templates#733
networktocode/ntc-templates#736

[ktbyers]

Looks good. Thanks for the contribution.