Read SYSTEM password from ENV ORACLE_SYSTEM_PASSWORD optionally

This is useful when running scripts and no user is around to enter
a system password value. In particular, for various CI server setups.

Usage Example:

```
ORACLE_SYSTEM_PASSWORD=secret rake db:migrate
```
Merge pull request #529 from Verdacom/allow-optional-env-variable-for-system-password

Read SYSTEM password from ENV ORACLE_SYSTEM_PASSWORD optionally