You can set most defaults for your Google Drive documents by opening the Page Setup dialog box, modifying the current settings, then clicking the “Save as Default” button. All future documents will be formatted as configured in the Page Setup menu. However, you can't disable the Auto-Indent feature in these settings.
To turn off the first-line auto-indent feature in Google Drive documents, open a document and slide the first-line indent indicator in the document ruler to the left. The first-line indent indicator is the small rectangle on the right side of the left indent indicator -- the small triangle pointing downward on the left side of the document ruler. Then align the first-line indent rectangle with the left indent triangle to disable he first-line auto-indent feature.