Some tentative improvements

[pierky]

Here I propose some changes:

• using the Docker networking API to create the bridge and to connect containers to it, instead of netns

• adding the --target-config-file to allow the target BGP daemon to use an existing configuration file that is on the host

• adding some arguments to config and bench to allow to set the local IP prefix and the local IP addresses of monitor, target and testers

• changing testers.testerX.tester in testers.testerX.neighbors

testers:
- name: tester1
  neighbors:
    10.10.0.10:
      ...
    10.10.0.100:
      ...
- name: tester2
  neighbors:
    10.10.0.20:
      ...
    10.10.0.200:
      ...

• adding ExaBGP MRT tester (based on mrtparse)

[pierky]

Damn, forgot to rebase. Will do ASAP.

[pierky]

Continuing on #18