SR-IOV: deprecate agent_required option

SR-IOV: deprecate agent_required option

When SR-IOV introduce in Juno Agent supported only link state change
Some Intel cards don't support setting link state, so to
resolve it the SR-IOV mech driver supports agent and agent less mode.
From Liberty the SR-IOV agent brings more functionality like
qos and port security so we want to make the agent mandatory.
(of course I already talked with Intel Guys to get their approval)

This patch deprecates the agent_required in Liberty
and updates the agent_required default to be True.

DocImpact
Closes-bug: #1488807
Change-Id: I8799425c2825415ef05bec909e6b4085ffc1e3c5