11.0.0.0-rc3: Odoo 11 needs to be run with Python 3

Due to some dependencies, Python 2 is also installed and it's the
default one in the image:

lrwxrwxrwx 1 root root       9 Dec 10  2015 /usr/bin/python -> python2.7*
lrwxrwxrwx 1 root root       9 Dec 10  2015 /usr/bin/python2 -> python2.7*
-rwxr-xr-x 1 root root 3546104 Nov 19  2016 /usr/bin/python2.7*
lrwxrwxrwx 1 root root       9 Mar 23  2016 /usr/bin/python3 -> python3.5*
lrwxrwxrwx 1 root root      16 Mar 23  2016 /usr/bin/python3-config -> python3.5-config*
-rwxr-xr-x 2 root root 4456240 Sep 18 15:59 /usr/bin/python3.5*
lrwxrwxrwx 1 root root      33 Sep 18 15:59 /usr/bin/python3.5-config -> x86_64-linux-gnu-python3.5-config*
-rwxr-xr-x 2 root root 4456240 Sep 18 15:59 /usr/bin/python3.5m*
lrwxrwxrwx 1 root root      34 Sep 18 15:59 /usr/bin/python3.5m-config -> x86_64-linux-gnu-python3.5m-config*
lrwxrwxrwx 1 root root      10 Mar 23  2016 /usr/bin/python3m -> python3.5m*
lrwxrwxrwx 1 root root      17 Mar 23  2016 /usr/bin/python3m-config -> python3.5m-config*