Add "duration" parameter to test case definition

Add "duration" parameter to test case definition

Add "duration" parameter to test case definition, in scenario:options
section. This parameter will be rendered in the traffic profile.

If the parameter is not present in the test case scenario options, the
default time written in the traffic profile options will be 30 seconds
(TrafficProfile.DEFAULT_DURATION = 30). If the traffic profile
injection time is not defined, the default injection time will be 30
seconds.

  testcase:scenario:options:duration (default = 30)
      render --> traffic_profile:duration
          parse --> TrafficProfile.duration (default = 30)

Target traffic profiles (RFC2544):
  - RFC2544Profile
  - IXIARFC2544Profile

JIRA: YARDSTICK-1194

Change-Id: I968922e6bb882d7ee15aa1c4db4037face7a3492
Signed-off-by: Rodolfo Alonso Hernandez <rodolfo.alonso.hernandez@intel.com>