EqualLogic cinder backend support

1) field-level validation beyond validates :presence => true isn't done yet
2) Wizard UI only supports single EQL backend; for multi-backend the advanced
   config view is required.
Not a huge reason, other than that's what we were already doing for the params that quickstack expected as arrays but we only ever added one value, so I just kept doing the same thing for consistency there. This could be easily refactored to work the other way (set as array type explicitly, and the entire value is ERB, and eqlx_pool be changed to return an array). We'll eventually need that anyway, once we update the wizard to configure multiple EQLX backends.