Switch test_db_url to a directory

Switch test_db_url to a directory

We must now set a dir (e.g. /home/opnfv/db) to dump results. It avoids
replacing "results" and prevent from limiting the use of this decorator.
All data will be dumped into url/dump.txt

As also proposed in FUNCTEST-729, missing directories are created too.
The decorator will raise exceptions if dir exists or if target is not a
directory.

Co-Authored-By: Cédric Ollivier <cedric.ollivier@orange.com>
JIRA: FUNCTEST-729
Change-Id: I9650c72493043539f17725f5c328c12639ecc059
Signed-off-by: xudan <xudan16@huawei.com>
Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>