Do not overwrite log files by default

[Didainius]

By default go-vcloud-director always overwrites log files. This causes problems in some libraries if they instantiate go-vcloud-director multiple times during single operations run. One example of that being Terraform where it runs plugin 5 times during one terraform apply command and part of the logs are instantly overwritten.

This PR changes default to "append" instead of overwriting and introduces environment variable GOVCD_LOG_OVERWRITE to allow enabling old behavior.

[lvirbalas]

Thank you.

[dataclouder]

Very good improvement!

[vbauzys]

at last :)