devstack plugin sometimes ask interactive user input

++/opt/stack/networking-midonet/devstack/plugin.sh:source:32 sudo /opt/stack/ne
tworking-midonet/devstack/midonet-pkg/install_pkgs.sh
Reading package lists... Done
Building dependency tree
Reading state information... Done
openjdk-8-jre-headless is already the newest version.
0 upgraded, 0 newly installed, 0 to remove and 90 not upgraded.
Reading package lists... Done
Building dependency tree
Reading state information... Done
zookeeperd is already the newest version.
Suggested packages:
  cassandra-toolsRecommended packages:
  ntp time-daemon
The following packages will be upgraded:
  cassandra dsc22 midolman midonet-cluster midonet-tools python-midonetclient
6 upgraded, 0 newly installed, 0 to remove and 84 not upgraded.
Need to get 178 MB of archives.
After this operation, 1634 kB of additional disk space will be used.
Get:1 http://builds.midonet.org/devstack/ unstable/main python-midonetclient all 2:5.2.0~rc1 [102 kB]
Get:2 http://builds.midonet.org/devstack/ unstable/main midonet-tools all 2:5.2.0~rc1 [43.7 MB]
Get:3 http://debian.datastax.com/community/ 2.2/main dsc22 all 2.2.7-1 [1316 B]
Get:4 http://debian.datastax.com/community/ 2.2/main cassandra all 2.2.7 [23.6 MB]
Get:5 http://builds.midonet.org/devstack/ unstable/main midolman all 2:5.2.0~rc1 [52.6 MB]
Get:6 http://builds.midonet.org/devstack/ unstable/main midonet-cluster all 2:5.2.0~rc1 [57.7 MB]
Fetched 178 MB in 49s (3556 kB/s)
(Reading database ... 81879 files and directories currently installed.)
Preparing to unpack .../python-midonetclient_2%3a5.2.0~rc1_all.deb ...
Unpacking python-midonetclient (2:5.2.0~rc1) over (2:5.2.0~rc1) ...
Preparing to unpack .../archives/dsc22_2.2.7-1_all.deb ...
Unpacking dsc22 (2.2.7-1) over (2.2.6-1) ...
Preparing to unpack .../cassandra_2.2.7_all.deb ...
Unpacking cassandra (2.2.7) over (2.2.6) ...
Preparing to unpack .../midonet-tools_2%3a5.2.0~rc1_all.deb ...
Unpacking midonet-tools (2:5.2.0~rc1) over (2:5.2.0~rc1) ...
Preparing to unpack .../midolman_2%3a5.2.0~rc1_all.deb ...
Unpacking midolman (2:5.2.0~rc1) over (2:5.2.0~rc1) ...
Preparing to unpack .../midonet-cluster_2%3a5.2.0~rc1_all.deb ...
Unpacking midonet-cluster (2:5.2.0~rc1) over (2:5.2.0~rc1) ...
Processing triggers for man-db (2.6.7.1-1ubuntu1) ...
Processing triggers for ureadahead (0.100.0-16) ...
Setting up python-midonetclient (2:5.2.0~rc1) ...
Setting up cassandra (2.2.7) ...

Configuration file '/etc/cassandra/cassandra.yaml'
 ==> Modified (by you or by a script) since installation.
 ==> Package distributor has shipped an updated version.
   What would you like to do about it ? Your options are:
    Y or I : install the package maintainer's version
    N or O : keep your currently-installed version
      D : show the differences between the versions
      Z : start a shell to examine the situation
 The default action is to keep your current version.
*** cassandra.yaml (Y/I/N/O/D/Z) [default=N] ?

Configuration file '/etc/cassandra/cassandra-env.sh'
 ==> Modified (by you or by a script) since installation.
 ==> Package distributor has shipped an updated version.
   What would you like to do about it ? Your options are:
    Y or I : install the package maintainer's version
    N or O : keep your currently-installed version
      D : show the differences between the versions
      Z : start a shell to examine the situation
 The default action is to keep your current version.
*** cassandra-env.sh (Y/I/N/O/D/Z) [default=N] ?