Caching svn credentials must not be activated by default

[Sorin Sbarnea <sorin.sbarnea@…>]

Currently bitten will mess your credential if you run bitten-slave on the same machine you are using for developing.

By default bitten should use --no-auth-cache when calling svn.

[hodgestar]

The current behaviour (i.e. not using --no-auth-cache) is actually quite useful for those of us using a separate user account to do builds and who don't want svn usernames and passwords stored in the bitten recipe (the main advantage here is that it means the recipe doesn't have to be secret and can, for example, be stored as is in version control system).