Fix ssh credential validation message
Fix ssh credential validation message

This error message specifies 'password' as one of the valid credential
keys, but really it should be 'ssh_password'. Fix it.

Change-Id: I5671f6f312afc142f6a58c68610b620edd3adf44

NaohiroTamura/ironic,SauloAislan/ironic,pshchelo/ironic,NaohiroTamura/ironic,pshchelo/ironic,openstack/ironic,openstack/ironic,SauloAislan/ironic