Running unit tests of keystoneauth in Python 3.5 reveals the issue
that in Python 3.5+, keys() is not a copy, and therefore items can't
be popped from it while iterating.
This patch iterate over copy of session.adapters keys in Python2/3.
Change-Id: I4a4340d6f0b09e047e992d0a7236f83ff5eac7a3
Closes-Bug: #1565728