Source lib/nova when upgrading tempest

In lib/nova is set variable NOVA_BIN_DIR which is later used for setting
cli_dir in tempest.conf. Without including lib/nova the variable is
evaluated as empty string.

Change-Id: I74f0cf8a3af287b072acefab92ac9a3facb443f5
Closes-Bug: 1391436
(cherry picked from commit d69386a2b78c53c48f734c48be15c217f9208981)