CSIT-1178: Prepare for bursty MRR

CSIT-1178: Prepare for bursty MRR

+ All suites and tests still refer to MRR instead of BMRR (or FRMOBL).
+ API change: Traffic should pass with maximum rate
  first argument got a default value,
  thus it needs to be moved towards the end of argument list.
+ Consequently, the value for it has been deleted from suites.
+ Test message is now compatible only with BMRR parsing.
- Reporting of transmit rate is left as TODO.
+ Global test __init__ with new way of giving defaults to global variables.
+ The new keyword is put into robot_enhancements.robot resource.
+ Number of trials and trial duration are now configurable.
- Bootstrap scripts have no support for overriding the defaults yet.

Change-Id: I4b108cc9c256730e62c03fbacba0a1c314f8ae88
Signed-off-by: Vratko Polak <vrpolak@cisco.com>