isisd, yang: unified lsp-timers command

isisd, yang: unified lsp-timers command

Yang constraints enforced by the northbound callbacks require that
the maximum lifetime be >= than (refresh interval + 300). When we are
moving from one config to another through frr-reload.py, we issue
a number of vtysh -c commands ('no lsp-refresh-interval level-1 500',
'no max-lsp-lifetime level-1 1000'), which reset these parameters to their
default values, respectively 900 and 1200. Depending on the actual
values in the current config, the order in which these commands are sent
might be the wrong one, in that we hit an invalid intermediate state and
make vtysh (and by extension frr-reload.py) return an error.

As a workaround, let's add a one-liner command that sets all these
inter-related parameters in one go, and make isisd display them as a
single line too, so that the diff will be computed as a single command.
The old individual commands are kept to ensure backwards compatibility.

Signed-off-by: Emanuele Di Pascale <emanuele@voltanet.io>