Add Database Variables group to releases

[m-s-]

We need the database credentials to drop the databases in the first stages of the releases, however the credentials are encrypted.

1. Add a new variable group "Database Variables" to the pipelines
2. Add instructions to the documentation on how to create this variable group in the Library

[joaoopereira]

implementation decision: Rename Docker Variables to Authentication Variables. This should have the following variables:

• Authentication Variables
• DatabaseOnlineUser
• DatabaseOnlinePassword
• DatabaseODSUser
• DatabaseODSPassword
• DatabaseDWHUser
• DatabaseDWHPassword