Allow user to override timeouts from the environment.

These are the variable names and their current defaults:

REPROTEST_TIMEOUT_SHORT=100
REPROTEST_TIMEOUT_COPY=300
REPROTEST_TIMEOUT_INSTALL=3000
REPROTEST_TIMEOUT_TEST=10000
REPROTEST_TIMEOUT_BUILD=100000

Signed-off-by: Holger Levsen <holger@layer-acht.org>