sos: Follow symbolic links with tree command

The VDSM SoS plug-in uses the tree command to get
a view into storage domains.  SDs can
contain symbolic links; however, tree does not
follow symbolic links by default.  This patch sets
an option on tree to follow symlinks so that the
entire SD may be assessed.

Change-Id: I5e09dcc9dc228be22f83ccb15a8aaae453001b7c
Signed-off-by: Keith Robertson <kroberts@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/12386
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-by: Douglas Schilling Landgraf <dougsland@redhat.com>