This change fixes bug introduced in https://lab.nexedi.cn/nexedi/slapos.buildout/commit/aba00feb5cc694be6da433d936e29c94f9ae0416 for develop.

As _install_setup_eggs is part of Base class, some code
from Custom.__init__ had to be moved to Base.__init__ as those were
required by _install_setup_eggs implementation as self.link, options['_e'],
self.newest and so on.

Without this change zc.recipe.egg:develop fail to process.