Modify gen_robot_ssh exception handling for py3
Modify gen_robot_ssh exception handling for py3

The symptom being addressed is the following:

UnboundLocalError: local variable 'login_exception' referenced before
assignment

For a line of code like this:

    except Exception as login_exception:

In py3, the login_exception is only in scope in the exception handling
block.

Change-Id: I59e00f427f93be7234dd180dfefb05bc55ded374
Signed-off-by: Michael Walsh <10404802c7de463f402c4a18638106ca1b12ef61@us.ibm.com>

openbmc/openbmc-test-automation,openbmc/openbmc-test-automation