allow to pass an (optional) username to the connection helper

Signed-off-by: Alfredo Deza <alfredo.deza@inktank.com>
add a global option to allow a different username

Signed-off-by: Alfredo Deza <alfredo.deza@inktank.com>
get_connection now will construct the username and host for you

Signed-off-by: Alfredo Deza <alfredo.deza@inktank.com>
install now uses the new username flag!

Signed-off-by: Alfredo Deza <alfredo.deza@inktank.com>