vfs_glusterfs: Enable per client log file

vfs_glusterfs: Enable per client log file

In Samba configuration file, one of the options of gluster type is
log file, the value of this option was not allowed to contain any
variables, as a result all the clients would have a single log file,
which complicated debugging.
In this patch, variable substitution is performed for gluster log file.
Hence allowing user to customise the gluster log file name.

Signed-off-by: Poornima Gurusiddaiah <pgurusid@redhat.com>
Reviewed-by: Ira Cooper <ira@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Tue Dec 17 23:44:16 CET 2013 on sn-devel-104