add APT hook to prevent proxmox-ve removal

add APT hook to prevent proxmox-ve removal

since this happens quite regularly when users accidentally install
conflicting packages.

sample output:
$ apt remove pve-qemu-kvm
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following packages were automatically installed and are no longer required:
  libxml-libxml-perl proxmox-widget-toolkit pve-edk2-firmware pve-i18n pve-xtermjs
Use 'apt autoremove' to remove them.
The following packages will be REMOVED:
  proxmox-ve pve-container pve-ha-manager pve-ha-manager-dbgsym pve-manager pve-qemu-kvm qemu-server spiceterm
0 upgraded, 0 newly installed, 8 to remove and 0 not upgraded.
After this operation, 37.6 MB disk space will be freed.
Do you want to continue? [Y/n] y
W: (pve-apt-hook) !! WARNING !!
W: (pve-apt-hook) You are attempting to remove the meta-package 'proxmox-ve'!
W: (pve-apt-hook)
W: (pve-apt-hook) If you really you want to permanently remove 'proxmox-ve' from your system, run the following command
W: (pve-apt-hook)       touch '/please-remove-proxmox-ve'
W: (pve-apt-hook) and repeat your apt-get/apt invocation.
W: (pve-apt-hook)
W: (pve-apt-hook) If you are unsure why 'proxmox-ve' would be removed, please verify
W: (pve-apt-hook)       - your APT repository settings
W: (pve-apt-hook)       - that you are using 'apt-get dist-upgrade' or 'apt full-upgrade' to upgrade your system
E: Sub-process /usr/share/proxmox-ve/pve-apt-hook returned an error code (1)
E: Failure running script /usr/share/proxmox-ve/pve-apt-hook

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>