Split BuildHooksEnv of LUs

Commit dd7f6776 added another call to BuildHooksEnv to provide
post-phase status variables. Since BuildHooksEnv also built the node
lists, that meant they have to be built twice. First a rather strict
check was used, but it turned out to be more tricky. Commit b423c513
had to remove the strict check again.

With this patch the function is split in two parts, one generating the
actual environment variables, and another part returning the node lists.
The former is called twice.

Unittests are updated.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>