Fix OS_CLOUD export for image validation

Fix OS_CLOUD export for image validation

Change fe2beb15bc9effb5b91ba17b520ab6c167dd6604 introduced a bug where
the CLOUD_CREDENTIAL_ID was being used for the OS_CLOUD export variable.
This was done since the test for multi-cloud image validation had the
credential IDs matching the OS_CLOUD variable. However, most of our
systems have the CLOUD_CREDENTIAL_ID set to the default of 'os-cloud'
and therefore after pulling updating to a global-jjb with that change
would no longer be able to validate images!

We now look for an OS_CLOUD variable in the cloud.cfg file and use it if
it is set, otherwise we default to an OS_CLOUD of 'vex' as we previously
did before change fe2beb15bc9effb5b91ba17b520ab6c167dd6604

Change-Id: I0af763495fa3a7ce2e8c080324abaf5d384e5188
Signed-off-by: Andrew Grimberg <agrimberg@linuxfoundation.org>