Patch  added option "no_track" to the keepalived's config file which
is generated by L3 agent in HA mode.
This was added to handle properly keepalived 2.x and interfaces which
are in DOWN state in the backup nodes.
But this "no_track" option is not compatible with keepalived 1.x series
which is available e.g. on Ubuntu 18.04.
As there is no easy way to check automatically if keepalived supports or
not this config flag, this patch introduces new config option
If this config option will be set to False, neutron L3 agent will not
add "no_track" to the keepalived's config.
As master branch is moving to gate on Ubuntu 20.04 where keepalived 2.x
is already available, this new config option default value is set to
(cherry picked from commit 7abe0ee34c)