iterate over subdirectories in order in find_egg_dir_for of bootstrap script, to ensure oldest vsc-install is picked

[boegel]

Testing of the bootstrap script is currently failing with (only) Python 2.7 because of a combo of a silly bug in the bootstrap script and the latest vsc-install checked into the master branch on GitHub (0.12.2) not being released yet on PyPI...

Two versions of vsc-install are being installed currently due to the vsc-install < 0.11.4 workaround that was added in #2717: vsc-install-0.11.3 and whichever is the latest version (which gets pulled in via vsc-base).

The 0.11.3 is supposed to take the upper hand, but that's currently not always the case apparently...

Depending on the order in which vsc-install installations are considered in the find_egg_dir_for in the bootstrap script, one or the other may be picked up. If the latest version is picked up, we're in trouble, especially if that version is not available on PyPI yet, since that results in an error like:

[[ERROR]] Failed to determine PyPI package URL for vsc-install: ...

The error has been improved here as well, to mention the pattern being searched for:

[[ERROR]] Failed to determine PyPI package URL for vsc-install using pattern '/(packages/[^#]+)/vsc-install-0.12.2.tar.gz#': 

Making sure that find_egg_dir_for considers the vsc-install installation in order (oldest first) fixes the problem.
In Python 2.7, os.listdir has a different default order than in Python 2.6, which explains why the problem only occurs there (at least in Travis).