Currently method "_ensure_config_dirs_exist" creates tftpboot/<uuid>
dir with wrong permission. This is due to the system umask setting
which overrides the default permission of 0777 to 0755 or 0750. When
the permission is 0750, BM can't get deploy_kernel and ramdisk from
tftpserver. This may happen only when tftp process is launched from
other user than root and as result can't read files created by Ironic.
So this patch tries to fix the issue by explicitly changing the
permissions defined in the config option ``[pxe]/dir_permission``.
Change-Id: I3119ec7ae31bf82f716bf082fa4c3296d6aa3587
Closes-bug: #1655568
