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
Use a compute service type from tempest.conf

Auto negative test generator is using static service type 'compute',
but EndpointNotFound happens when customizing compute service type
on Keystone.
This patch fixes the problem by using a compute service type from
tempest.conf.

Change-Id: I5fcf53cc9a24bf4f2a00f11540bd87932212933f
Merge "Use a compute service type from tempest.conf"
Merge "Moved init_conf call and preserve tempest network objects"