ospfd, doc, tests: combined SRGB/SRLB command

ospfd, doc, tests: combined SRGB/SRLB command

similarly to what was done for IS-IS in commit 01d43141, combine
the SRGB and SRLB commands for OSPF-SR, so that we can replace
overlapping ranges in one sweep change.

Also allow the range configuration to be stored before SR is enabled.
There is no reason why we should not - in fact that constraint meant
that we were always requesting the default label ranges regardless
of what we actually wanted to use.

Finally, update the topotests now that we do not need to refresh
the SRGB/SRLB/MSD after disabling SR. Note that the prefix-sid still
needs to be re-added.

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