replace type() to isinstance() in nova

In nova/utils.py, a func use the "type" method to
determine the type. It's bertter to use the "isinstance"
method instead.this bug is fixed in glance:
https://review.openstack.org/#/c/65611/4

Change-Id: I10d25d2308436239c50cf3932c212e22c45f92a8
Closes-Bug: #1267300
Merge "replace type() to isinstance() in nova"