pki: use relative lock file

Current implementation uses /var/lock/ovirt-engine/* to perform lock of
pki operations.

Problem in current implementation is that /var/lock may be cleared on
distributions so no ovirt-engine exists. Also, in development mode there
is no access to /var directory.

The new implementation uses persistent SignReq.lock file which is
relative to SignReq.sh at PREFIX/etc, which is a valid location.

Change-Id: I4274778254167db8f632908a9bdcc0d6ad284b3e
Signed-off-by: Alon Bar-Lev <alonbl@redhat.com>