Add check and waiting for exabgp http server

[bingwang-ms]

Description of PR

Summary:
Fixes # (issue)
Add check and waiting for exabgp http server to get ready.

Type of change

• Bug fix
• Testbed and Framework(new/improvement)
• Test case(new/improvement)

Approach

What is the motivation for this PR?

The exabgp plugin has a problem in handling 'started' state:
The start_exabgp method only checks the status returned by supervisorctl,

def start_exabgp(module, name):
    refresh_supervisord(module)
    exec_command(module, cmd="supervisorctl start exabgp-%s" % name)

    for count in range(0, 60):
        time.sleep(1)
        status = get_exabgp_status(module, name)
        if u'RUNNING' == status:
            break
    assert u'RUNNING' == status

However, the http service (implemented in http_api.py) may be not ready at that time. As a result, the following call announce_route raises an exception.
Actually, it takes more than 20 seconds for http server gets ready after running http_api.py in vtestbed.

How did you do it?

Add an End-to-End check and wait after exabgp('started'), until http server is ready.

How did you verify/test it?

Run test_bgp_speaker.py and test_announce_routes.py in my vtestbed.

Any platform specific information?

No.

Supported testbed topology if it's a new test case?

No.

Documentation

No.

[wangxin]

Please align the code and remove extra spaces around =.

[wangxin]

Please add argument module_ignore_errors=True. Otherwise this step will raise exception after PR #1786 is merged.

[bingwang-ms]

Done.
Thank you so much!