plugin: simplify and fix create-base-path vs mkdir logic

plugin: simplify and fix create-base-path vs mkdir logic

In the previous code, if `create-base-path` was explicitly
set to false, it would be treated the same as if it was
undef, falling through to whatever 'mkdir' was.

Instead, the new options should always be preferred, and the
logic can be simplified to a single line.

Here's the table showing the difference, 'u' being 'undef':

config: mkdir:  u 0 1 u 0 1 u 0 1
        create: u u u 0 0 0 1 1 1
        =========================
mkpath: old:    1 0 1 0 0 1 1 1 1
        new:    1 0 1 0 0 0 1 1 1

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>