deprecate mkdir option for create-base-path and create-subdirs

deprecate mkdir option for create-base-path and create-subdirs

The `mkdir` option has two meanings[0][1] which are split up in `create-path`
and `create-sub-dirs`.

The `create-base-path` option decides if the path to the storage is
automatically created or not.
The `create-subdirs` options decides if the default directory
structure (dump, images, ...) at the storage location is created.

The `mkdir` option is still working but will trigger a warning in the
logs.

As a side effect, this also fixes #3214 because the `create-base-path` option
is now run after the `is_mountpoint` check in the `activate_storage`
method in DirPlugin.pm.

The 'mkpath' command has been moved into a new helper function that
first determines if the conditions to create the path is true, called
'config_aware_base_mkdir'.

[0] https://lists.proxmox.com/pipermail/pve-devel/2020-December/046575.html
[1] https://lists.proxmox.com/pipermail/pve-devel/2020-December/046576.html

Signed-off-by: Aaron Lauterer <a.lauterer@proxmox.com>
Reviewed-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>