Change remaining 'savanna' references in sahara/tests

These changes remove the remaining 'savanna' references under the
'tests' dir to 'sahara' and modify necessary files outside of
'tests' to make unit and integration tests pass.

* Change member savanna_config to sahara_config for alembic
* Change description strings in sahara/resources
* Change exception strings in plugin validation
* Change tests/README
* Change param names, defaults, and help in integration tests config.py
* Change alembic migration test lock prefix
* Change SAVANNA_MYSQL_PRESENT and SAVANNA_TEST_POSTGRESQL_PRESENT in
  alembic tests
* Change 'Generated by' message in crypto.py

Partial-Implements: blueprint savanna-renaming-service
Change-Id: I11e1bab1fce76a411a8659b947e7bae90ea521a9