Clean requirements.txt
Clean requirements.txt

It removes all requirements which are not detected by pipreqs [1]
except:
  - robotframework-httplibrary
  - robotframework-requests==0.4.7
  - robotframework-sshlibrary==2.1.3

They seem indirectly required for the next ODL suites: old_netvirt
and fds.

It also defines pbr as requirement and orders dnspython3 conditions.

[1] https://github.com/bndr/pipreqs

Change-Id: Idcd01296e5a5862ff8528dc083468b5c4905c9f9
Signed-off-by: Cédric Ollivier <d48310251a4a484d041bc5d09a9ac4d86d20f793@orange.com>

mywulin/functest,opnfv/functest,mywulin/functest,opnfv/functest