Store created nova client in OpenStackClients.

All created clients are stored in the instance of OpenStackClients
which is associated with a stack - this prevents clients from
being repeatedly created throughout the request.

Commit f3fc5c5 introduced a regression which stopped storing the
nova client. This change addresses that regression.

Change-Id: I60088b60f623aac11e473d185f2924d4f4bd1cfe
Merge "Store created nova client in OpenStackClients."