Do git clean in autorelease-checkout-gerrit-patch

Do git clean in autorelease-checkout-gerrit-patch

For a patch which removes a submodule (project leaving autorelease)
checking out HEAD before autorelease-checkout-gerrit-patch
was causing the project directory be present during verify-autorelease
which could lead to false Verified+1.

Recent example: Removing Yangtools
without updating validate-pom.xml properly.

This change adds forceful clean to autorelease-checkout-gerrit-patch.
The jacoco workaround now has to be applied after such cleaning.

Change-Id: I651a7eaa212835d6cce52ee3706a85f12189f244
Signed-off-by: Vratko Polak <vrpolak@cisco.com>