Fix #303: Support creating and dropping a PostgreSQL database when using pg8000 driver

[mohaseeb]

As reported by #303 current implementation of create_database/drop_database doesn't support creating/dropping a PostgreSQL database when connecting using pg8000 driver. The reason is when pg8000 driver is used (i.e. when the drivername is postgresql+pg8000), create_database/drop_database doesn't set the isolation level needed to execute the 'create database'/'drop database' command on PostgreSQL. This PR is fixing that.

• Commits 5dec6f2 and ebd6b10 add a TC that tests creating a PostgreSQL DB using pg8000. Only the TC without the fix was added first so the travis-ci job fails to reproduce the error (see the TC failure here).
• The following commit adds support to create a PostgreSQL DB using pg8000.
• Commit 49ba768 extends the tests to test dropping a PostgreSQL DB using pg8000. The TC fails as intended to reproduce the error as can be seen here.
• The following commit adds support to drop a PostgreSQL DB using pg8000.

[mohaseeb]

@kvesteri this PR is ready for review, please have a look when possible. Thanks!

[kvesteri]

Thanks for the PR! 🚋