Add Big IP exceptions metrics, fix logging add retries for sync RouteDomain.

[velp]

This PR adds several fixes for l2_sync problems:

1. Fixes for attribute error in response logging handler:

   ERROR octavia_f5.controller.worker.controller_worker AttributeError: 'Response' object has no attribute 'txt'
   

2. Adds retries for EnsureRouteDomain task to avoid situation when route domain was not configured in first attempt
3. Adds several metrics for low-level Big IP REST client to monitor exceptions in l2_sync flows and to be able to see exact number of failed requests.

Related to issue: https://github.wdf.sap.corp/cc/octavia-issues/issues/39

[m-kratochvil]

LGTM. Thanks for following up on this topic, Vadim!

[BenjaminLudwigSAP]

LGTM, thank you!