command: making default log file location generic in help text

The default name of log file contains an execution timestamp;
the command help text prints it evaluating it at command
execution time so that file will not really exist.
Making it generic using a placeholder.

Change-Id: I1755c4aa60340be5fb95b7bfba789b4dd33b2633
Bug-Url: https://bugzilla.redhat.com/1061636
Signed-off-by: Simone Tiraboschi <stirabos@redhat.com>
(cherry picked from commit 73da03974029086ead9747d69c023c91939aa6d2)