Only append a trailing slash if there are only two in the URL given

Otherwise we may end up with an URL such as:
  driver://user:pass@host:port/dbname/
which is invalid, the trailing slash should not be there.
Display to the user the actual error message this it is informative

The actual error message contains information such as:
- You need to provide a password
- Password was invalid
- Server is not running
- Host could not be found

These information are more helpful to debug a problem than the standard
error message in place now.
Do not ask for the database name if it is already in the URL provided

If the user provided an URL of type:
  driver://user:pass@host:port/dbname
Then we can extract the database name from the URL and avoid asking the
user again.
Adjust the ``except`` commands to be python3 compatible
Fix incorrect configuration file after init

Url and stellar_url data in stellar.yaml configuration file
data should not include database name.
Merge branch 'psql_enhancement'