[vdsm] merge rights on master branch

Nir Soffer has been contributing to Vdsm for more than two years now. Beyond his storage-related expertise, he is well-known for his thorough code reviews and his pains when the master branch is broken. It's time for him to take even bigger responsibility. David, please grant Nir Soffer with merge rights on the master branch of Vdsm. I'll keep merging network, virt, and infra patches myself, but much less than before. Adam keeps his merge right as before (he tends to use them on emergencies only). Regards, Dan.