We document that this can be disabled by setting the
[pxe]tftp_master_path config to "<None>", but don't
actually support it in code. oslo.config doesn't actually
translate that value to the Python None as we expect.
Allow disabling the cache by setting the config to the empty
string, as in "tftp_master_path=". This doesn't make sense as
a directory to use as a cache anyway, so it shouldn't break anyone.
Change-Id: Icc7d08ae47e0e450a612c922ae3d665c56880262
Story: 2004608
Task: 28506
4c987aa51b