pmgversion: be compatible with minimized container installation

pmgversion: be compatible with minimized container installation

One can now install PMG in a CT envrionment through the new
proxmox-mailgateway-container meta package, but this wasn't correctly
checked by the API PMG versions code.

Add the new meta package as optional one, and if it is installed
replace the bare-metal meta packge info with it, both can never be
installed at the same time. Also move pve-firmware to the optional
package list.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>