extracted methods download_binary_cache(), update_binary_cache(),
_pre_destroy()

Conflicts:
	slapos/cli/register.py

this retains comments and avoids issues with unicode, multiline values
(certificates) etc.etc.

both ipython and bpython are built and provided with slapos. bpython is the
default now, with colorful drop down menus. we can choose later.

also removed related tests