fix debian runit init script type

previously debian fell through by accident to the sysvinit
method, which broke when we started assigning all of the "debian"
platform_family to be upstart.

since most derived-distros that i'm aware that ohai detects are
descendents of ubuntu rather than debian i structured the case
statement to fall through by default to ubuntu-like behavior.
Merge branch 'lcg/fix-debian'