OpenStack credentials -- username and tenant name should be separate fields

[n1zyy]

Deltacloud wants the OpenStack username and tenant name as "username+tenantname", but that doesn't mean we should be collecting the information this way, especially not in a box just labeled "Username."

We should really split this out into two fields. Failing that, we at least need a label indicating what we're doing to the username, but that's not great.