Commit 7020491 ("esxi: add 'port' config parameter") started using
the 'port' option in a second plugin, but the definition stayed in the
PBS plugin. Avoid the hidden dependency and move the definition to the
base plugin instead.
It is necessary to mark it as optional or it would be required always.
Clarify that the option is not used by NFS and CIFS.
Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
