Bugfix: port_security_enabled issue

Bugfix: port_security_enabled issue

JIRA: YARDSTICK-765

When port_security_enabled is not set, VMs are assigned with security group 'default'.
When using 'default' security group, all egress traffic and intercommunication in the
default group are allowed and all ingress from outside of the default group is dropped
by default (in the default security group). This causes yardstick cannot ssh into VMs.

If port_security_enabled is not set, we should still add the security group that created
by yardstick to the VMs.

Change-Id: Ifd22fb452e0077581b6900f8f51c4e3c342a30aa
Signed-off-by: JingLu5 <lvjing5@huawei.com>