behave_tests: refactor TestAPI DB lookup

- use testapi.TestapiClient everywhere
- relax search constraints: match only project name (nfvbench), test
  case name (characterization or non-regression), scenario tag
  (throughput or latency) and user_label (test chain identifier:
  identifies, among other things, the platform, the compute class under
  test, ...)
- add unit tests for some of the related behave steps

Change-Id: I26763f845c2286601cb958b326525b29320a1627
Signed-off-by: Gwenael Lambrouin <gwenael.lambrouin@orange.com>