Neutron-metadata-agent can cause big load on the RPC bus and
neutron-server by asking for port details very often. And this can be
optimized by simple using cache mechanism provided by oslo.cache module.
This feature wasn't really described in our docs so this patch adds
short document about why and when use cache in metadata agent, why it's
not needed in the neutron-ovn-metadata-agent and how to enable it.
Closes-Bug: #2024581
Change-Id: I2c7e496f4c0588eebc1fbf42a43473101f67032f
49b68d36a0