improve devices connection methods in tests

- improve log parser output
- move some constants to common/test_utils:
  - RESTconf base URL
  - ODL and nodes credentials
  - json and XML application-type headers
- improve functions to mount or unmount devices onto ODL
- centralize them in common/test_utils to use the same ones in all tests
  and speed up tests by removing some timers

TODO in next other changes:
- use test_utils get/post/del/put_requests methods in all tests
- replace the other timers with the log parser
- find an alternative to calling 'tail -f' host command
- address pylint issues

Change-Id: I2b97700e363a3773dcc1606448eedae57c8c6696
Signed-off-by: guillaume.lambert <guillaume.lambert@orange.com>