The fixed for allowing centos 7 container to connecting to nfs server was :

lxc.aa_profile: unconfined

As advised by https://tweenpath.net/fix-on-connecting-to-nfs-server-from-proxmox-centos-7-container/

But apparently for proxmox-ve 5.2-2 it will get your container to unable to start. This is due to lxc.aa_profile was renamed to lxc.apparmor.profile. The solution for this is :

lxc.apparmor.profile: unconfined