doc: Added dumpling installation for Debian/Ubuntu.

Signed-off-by: John Wilkins <john.wilkins@inktank.com>
doc: Minor tweaks to debian install.

Signed-off-by: John Wilkins <john.wilkins@inktank.com>
doc: Added dumpling to RPM installation.

Signed-off-by: John Wilkins <john.wilkins@inktank.com>