pldm_visualise_pdrs.py: make --user optional
pldm_visualise_pdrs.py: make --user optional

Most ssh clients (ssh, scp, etc) will default to the currently logged in
user if unspecified elsewhere, including paramiko itself.

Make --user optional and default to the currently logged in user to be
consistant with the behavior of other ssh client applications.  This
also avoids the unusual sitation where --user is only required if it
does not appear in an ssh configuration file host entry (useful if
support for parsing ssh configurations were to be added in the future).

Change-Id: I0d4fdd51d03c205b1457ea199934fda2e3a2ab29
Signed-off-by: Brad Bishop <713d098c0be4c8fd2bf36a94cd08699466677ecd@fuzziesquirrel.com>

openbmc/pldm,openbmc/pldm,openbmc/pldm