Fix parsing of log-levels by removing date/time prefix

[bengesoff]

The TF_LOG and TF_LOG_PROVIDER environment variables should be able to filter the provider logs based on the log level (DEBUG, INFO, WARN, ERROR etc). However, the default logger in the log package prepends the date and time to each log line. This breaks the parsing of the logs in Terraform core, as it expects each line to start with the log level in square brackets. Instead, it finds the date and defaults every line to INFO regardless of the intended severity. An example of how this line is outputed is as follows:

2021-07-08T10:41:00.615+0100 [INFO]  provider.terraform-provider-fastly_v0.32.0-5-gba6a3dbd: 2021/07/08 10:41:00 [DEBUG] Refreshing WAFs for (1F5jhDFJKHKhdfjDF0On28Pj7): timestamp=2021-07-08T10:41:00.615+0100

Here you can see that we intended to log as a DEBUG level, but it ended up coming out as INFO. The timestamp is also duplicated everywhere.

To fix this, the default logger can be configured to skip prepending the timestamp using log.SetFlags(0). With this change, the log line now becomes:

2021-07-08T10:48:23.189+0100 [DEBUG] provider.terraform-provider-fastly_v0.32.0-6-g6e80f288: Refreshing WAFs for (1F5jhDFJKHKhdfjDF0On28Pj7): timestamp=2021-07-08T10:48:23.189+0100

Setting the TF_LOG environment variable to INFO also means that this line prevented from being outputted. This should help to filter the logs better in some situations.