trac-admin initenv should continue if existing directory is empty

[mdorey@…]

If the directory for the Trac environment exists before trying to initialize it Trac fails with the error:

Creating and Initializing Project
Failed to create environment. [Errno 17] File exists: '/home/software/trac-root/salma-hayek'
Failed to initialize database. 1

If the existing directory is empty Trac should proceed to create the environment. This is consistent with svnadmin create which will proceed unless the directory exists and is not empty.

Also with the interactive initialization it would be good to make this check once before the questions so that it can fail sooner.

[mgood]

Cleaned up the decription.

Please try to keep tickets a bit more concise. Two full installation logs aren't really necessary to descript the topic here.

[mgood]

This is the same as #3055 which has already been implemented.