Support for multiple hosts in PostgreSQL connection string

Support for multiple hosts in PostgreSQL connection string

Provide support for multiple hosts in the PostgreSQL connection string.

A user requested for SQLAlchemy to support multiple hosts within a PostgreSQL URL string. The proposed fix allows this. In the event that the url contains multiple hosts the proposed code will convert the query["hosts"] tuple into a single string. This allows the hosts to then get converted into a valid dsn variable in the psycopg2 connect function.

This pull request is:

- [ ] A documentation / typographical error fix
	- Good to go, no issue or tests are needed
- [X ] A short code fix
	- please include the issue number, and create an issue if none exists, which
	  must include a complete example of the issue.  one line code fixes without an
	  issue and demonstration will not be accepted.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.   one line code fixes without tests will not be accepted.
- [ ] A new feature implementation
	- please include the issue number, and create an issue if none exists, which must
	  include a complete example of how the feature would look.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.

**Have a nice day!**
Fixes: #4392
Closes: #5554
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/5554
Pull-request-sha: 3f7a0ab8df9f1411a9f1ac0e152583bc7bf0c365

Change-Id: I3f3768d51b8331de786ffdc025b7ecfc662eafe5