bgp: T9407: do not render "bgp reject-as-sets" when it is the FRR default

Since FRR 10.5 "bgp reject-as-sets" is the default. FRR does not print
the default in show running-config, so frr-reload never finds the line in
the running config and sends it again on every reload. The command
handler resets every BGP session of the instance (NOTIFICATION Cease,
other configuration change) whether or not the value changed,
so with "protocols bgp parameters reject-as-sets" set, every commit that
reloads FRR reset every established session, not only commits that touch
BGP.

Render only the non-default form: "no bgp reject-as-sets" when the
option is not set, as T7664 added it, and nothing when it is set. The
option is a valueless leaf, so it has no XML default to read; the
rendered "no" form already was the only place that knows what FRR's
default is. Changing the option still reaches FRR: frr-reload sends
"bgp reject-as-sets" when the option is set (the "no" line leaves the
rendered config) and "no bgp reject-as-sets" when it is deleted, and
each change resets the sessions once, as before.

Add a smoketest that reads the rendered /run/frr/config/vyos.frr.conf,
because FRR never prints the default form and vtysh cannot show it.

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