pki: use local lock file

remote file systems do not always support file locking using flock.

use lock file within /tmp to ensure local file system usage.

this has the side effect of leaving leftover file.

the alternative is to use symlink primitive lock loop which is more
complex.

Change-Id: I1d82032defcbcf7248ae24092deaf061ce7905fd
Signed-off-by: Alon Bar-Lev <alonbl@redhat.com>