Make application of dconf settings more reliable

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>
Don't auto-update the test image

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>
Tweak test Vagrantfile to allow testing of all the images

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>