Moved init_conf call and preserve tempest network objects

Fixes race condition where configuation is accessed
before it is configured.

Preserve ports and subnets for tempest.conf networks.

Devstack creates private and public networks, and ports
and subnets in these networks. Cleanup needs to preserve
these objects unless --delete-tempest-conf-objects flag
is used.

Also removed unnecessary argument and updated doc string.

Change-Id: I30381191fde36aa71639a4aac3a4ca18a0733052
Merge "Moved init_conf call and preserve tempest network objects"