Remove unused config option image_alt_ssh_user
Remove unused config option image_alt_ssh_user

It's not used in the code and the corresponding
image_alt_ssh_password is no longer there so remove the option.

Change-Id: I9d3b3cd6e481edf4c1092335736acc04f1127adb

Juniper/tempest,LIS/lis-tempest,masayukig/tempest,vedujoshi/tempest,masayukig/tempest,cisco-openstack/tempest,zsoltdudas/lis-tempest,Tesora/tesora-tempest,bigswitch/tempest,Juniper/tempest,openstack/tempest,vedujoshi/tempest,sebrandon1/tempest,sebrandon1/tempest,Tesora/tesora-tempest,openstack/tempest,LIS/lis-tempest,bigswitch/tempest,cisco-openstack/tempest,zsoltdudas/lis-tempest