Fix py3 issue in get_undercloud_registry()
Fix py3 issue in get_undercloud_registry()

In python3 subprocess returns <class 'bytes'> type,
in python2 this was <type 'str'>. Add universal_newlines
argument to subprocess.Popen() call to ensure we get a
string.

Related: RHBZ#1762303
Closes-Bug: #1849140
Change-Id: I6fb51796d6c6b2efa82a0e96fa954f479a1641f2

openstack/tripleo-common,openstack/tripleo-common