Usually, pre-emptive strikes require significant political support, based on both solid evidences of an emerging threat and consensus among the vast majority of states involved.
what is the procedure for such an intervention?
which countries have the legal right to attack North Korea […]?
The most legitimate way is probably establishing an international coalition under the auspices of the UN.
This requires adoption of a resolution by the UNSC (UN Security Council).
It is well-known that Russia is systematically (ab-)using its veto power to block the UNSC resolutions about Korea. During the history of UN, the Russia has vetoed five UNSC resolutions about the "North Korea".
One of the most vivid story what happens if the Russians are not using its veto power to cover up the "North Korean" aggression, is maybe January 1950, when the "Soviet" Russia's representatives boycotted UN meetings. In the absence of them, the UNSC was able to vote for the intervention of a UN-mandated force to defend Korea "to repel the armed attack" from the North.
According to "Telegraph Agency of the Soviet Union" (TASS), even today the Russia still considers its right to veto a ‘cornerstone’ of UN Security Council’s architecture.
At the moment, the Russia has three military fronts (two in Ukraine and one in Syria) and several frozen conflicts (Chechnya, Dagestan, Georgia, Moldova, and Azerbaijan/Armenia). The Russian armed forces are spread between these fronts, so they desperately need some political resources to keep control over all these conflicts.
It seems to be hard to accomplish that if yet another armed conflict arises.
So in my understanding, Moscow simply informs Pyongyang that it can no longer use its veto power, nor can it provide with military help in case if the rest of the world decides to arrange a military intervention to the "North Korea".
Logically, this declaration can also serve as a signal to other UNSC members that Russia is not willing to block adopting further resolutions about this topic.
This becomes even more obvious, since recently (2/Mar/2016) the Russia has not blocked UNSC adoption of a new resolution encompassing sanctions against North Korea, "the strongest U.N. sanctions in 20 years."
would there be a battle for glory […] of "liberating" North Korea from its regime?
It's a bit philosophic question. There is no such term as "battle for glory" in politics. Even more, liberating of a nation from its regime is not an internationally accepted action. In general, even the most blood-hungry regimes are being tolerated, unless they attack other states. For example, the "Soviet" Russia was tolerated regardless of huge population losses. To compare, the total losses of native Ukrainians during only 1921-1945 are estimated to be 30-40 million (1). Nevertheless, no states were keen to "liberate Ukrainians from the Russian regime".
What the coalition can do is defend other states from the emerging threat of "North Korean" attacks by destroying any weapons that can "reduce" someone else "to seas in flames and ashes in a moment".
(1) As per comment below, Ukraine underwent 6.8 million population loss during WWII. Obviously, Russia was not solely responsible for this entire number.