Allow RPM building with uncommitted changes

The release script didn't allow building RPM packages with uncommitted
changes because it ran "git checkout ." afterwards to revert the version
and changelog changes.
This check was removed and now only the version.rb and RPM_CHANGES files
are reset after building the RPM.
Allow RPM building against different projects

Allow specifying the api, project and build target when running
rpm:build, for example:
`rake rpm:build["https://api.opensuse.org","systemsmanagement:machinery","openSUSE_13.1"]
Build and install the RPM package

Instead of synchronizing the current git tree for integration tests
the rpm package of the current code is build and installed in the VM.
Adapt integration tests to use the RPM machinery

The machinery vms have now the RPM package installed so the path to call
machinery needed to be adapted.
Remove unnecessary gems from postinstall_machinery.sh

These gems shouldn't be needed in the integration tests images, only outside.
Remove GIT repositories from postinstall_machinery.sh
Merge pull request #34 from SUSE/use_rpms_for_integration_test2

Use rpms for integration test2