checkbox-ng: mock away dbus if MOCK_DBUS=yes

This somewhat crazy patch makes 'import dbus' and work well enough so
that dbus_ex and thus checkbox service "works" in testing. There are no
real tests for any of the dbus code so this, at least, allows us to test
checkbox-ng.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>
.travis.yml: test checkbox-ng, checkbox-support, providers

This patch makes our travis configuration much more elaborate, enough to
test all of the python source code and all of the providers.  One of the
providers (client certification) is not enabled as it has been found to
be buggy.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>
"automatic merge of lp:~zkrynicki/checkbox/travis-more/ by tarmac [r=sylvain-pineau][bug=][author=zkrynicki]"