Skip testcases when shared nets are found in AutoAllocateNetworkTest

Shared nets is common configuration for the system, e.g., if we set
create_isolated_networks = False, we will create a shared net for
all testcases/tenants to use, which is specified by fixed_network_name,
 cfg.StrOpt('fixed_network_name',
            help="Name of the fixed network that is visible to all test "
                 "projects. If multiple networks are available for a "
                 "project, this is the network which will be used for "
                 "creating servers if tempest does not create a network or "
                 "a network is not specified elsewhere. It may be used for "
                 "ssh validation only if floating IPs are disabled."),

So, when shared nets are found, we should skip the testcase instead of
raising an exception.

Change-Id: I44aa06c1e0b5fb1d58707605ca084b272223aaf7
1 file changed