Add default link parameter support for ethtool module

Add default link parameter support for ethtool module

Ticket: CM-5254
Reviewed By: roopa
Testing Done: tested master and 2.5_br images with testifupdown2 suite and hand tested

This patch creates a json defaults file upon bootup
(which can be overridden by customer configs in /etc)
which the ethtool module in ifupdown2 will consult
when "link-x" configs are removed in order to restore
them to the initial settings used by the switch.
(cherry picked from commit 8388664f5a5a85f2a813cafbf40ac92d7b86f4bf)

Conflicts:
	packages/cl-utilities/usrlib/update-ports
(cherry picked from commit 21c9c10ab2fccaf60be9accb337e82541d497cc4)