Import "traffic_profile" modules only once

Import "traffic_profile" modules only once

"traffic_profile" modules should be imported only once. Every time
TrafficProfile.get is called, the modules under
"yardstick.network_services.traffic_profiles" are loaded [1]. Instead
of this, the modules should be registered only once the first time
"yardstick.network_services.traffic_profiles.base" is loaded. This
will reduce the execution time and will avoid unnecessary calls.

[1] https://github.com/opnfv/yardstick/blob/d2c7cc4e9768ed003257a95c92cdb278d516761b/yardstick/network_services/traffic_profile/base.py#L36-L37

JIRA: YARDSTICK-951

Change-Id: Ia3565378ba3a1377fcb0aea8bda50ef8189414fd
Signed-off-by: Rodolfo Alonso Hernandez <rodolfo.alonso.hernandez@intel.com>