dockertest.xceptions: Inherit all docker exceptions from AutotestError

This is essentials for subSubCaller tests to distinguish between
unknown failure and know docker failure. (first one should fail the
test immediately, the second one should fail one subSubTest and
proceed with the next one.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
Merge pull request #45 from ldoktor/framework

dockertest.xceptions: Inherit all docker exceptions from AutotestError