inventory: do not name a variable "vars"

vars is a builtin that returns the __dict__ attribute of an object.
Do not shadow it, instead rename the variable now that it has been
split out of a function that already used "group_vars" for something
else.

Suggested-by: Erik Skultety <eskultet@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>