Update default staging target to Xenial

[eloquence]

The securedrop_staging_install_target_distro is currently set to trusty in
https://github.com/freedomofpress/securedrop/blob/develop/install_files/ansible-base/group_vars/all/securedrop . This needs to be changed to xenial as part of #4155.

[eloquence]

In building the staging environment for Xenial with the virtualbox provider, I encountered an error during provisioning (it failed to locate the trusty packages, since the packages built during the make build-debs step were for xenial) that I traced down to this variable. What's not clear to me is why this error occurred at all, since it appears that we override this variable both for libvirt and virtualbox:

https://github.com/freedomofpress/securedrop/blob/develop/molecule/virtualbox-staging-xenial/ansible-override-vars.yml

https://github.com/freedomofpress/securedrop/blob/develop/molecule/libvirt-staging-xenial/ansible-override-vars.yml#L9

Regardless, it clearly shouldn't be trusty, so I'll submit a PR to fix.