poll_email.py: handle 'too many connections' error at login (#1019)

This commit, if applied, adds the logic to retry when the IMAP login fails.

It implements the exponential backoff.

This commit fixes an error that occur when there are a significant number
of concurrent jenkins job running:

Traceback (most recent call last):
  File "./tests/jenkins/downstream/poll_email.py", line 111, in <module>
    args.timeout))
  File "./tests/jenkins/downstream/poll_email.py", line 65, in wait_email
    M = connect()
  File "./tests/jenkins/downstream/poll_email.py", line 54, in connect
    M.login(username, password)
  File "/usr/lib64/python2.7/imaplib.py", line 520, in login
    raise self.error(dat[-1])
imaplib.error: [ALERT] Too many simultaneous connections. (Failure)