Replace git.openstack.org URLs with opendev.org URLs

Thorough replacement of git.openstack.org URLs with their opendev.org
counterparts.

Change-Id: I97409815d943ade7adce00168066ca6275bfd68a

README.rst

@@ -18,4 +18,4 @@ Script for creating Fedora and Ubuntu cloud images with our elements and default
 
     tox -e venv -- sahara-image-create
 
-Note: More information about script `diskimage-create <https://git.openstack.org/cgit/openstack/sahara-image-elements/tree/diskimage-create/README.rst>`_
+Note: More information about script `diskimage-create <https://opendev.org/openstack/sahara-image-elements/src/branch/master/diskimage-create/README.rst>`_

tox.ini

@@ -5,7 +5,7 @@ skipsdist = True
 
 [testenv]
 usedevelop = True
-install_command = pip install -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt} {opts} {packages}
+install_command = pip install -c{env:UPPER_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/raw/branch/master/upper-constraints.txt} {opts} {packages}
 setenv =
     VIRTUAL_ENV={envdir}
 deps =