Exclude remote file systems from inspection of unmanaged files

Remote file systems are all file systems which are mounted either via NFS or
CIFS, directly or via the auto mounter. These file systems are not extracted
by the unmanaged-files inspector. The mount points of remote file systems are
stored as entry of type 'remote-dir' in the unmanaged-files scope in the
system description.
Use tag remote_dir instead of remote-dir

to be consistent with the rest of the manifest file.
Make sure that one-liner unmanaged-files output have a break
Add remote_dir type information to migration description
Prevent remote_dirs from being also registered as dir

The unmanaged-files inspector listed the remote-dirs also as
regular dirs, at least some times.
This commit does prevent it.
Add and activate NFS server to base images

We want to have integration tests for remote path filtering.
To test these a running NFS server environment is set up in our base
images.
Add integration test for unmanaged-files remote-dirs

and fix issues in the integration test caused by the new remote_dir tag.
Merge pull request #276 from SUSE/remote_file_system3

Remote file system3
Add integration test that inspects and builds a SCC registered system.

Inspect a SLES12 SCC registered system and build a new image using the
inspected data.
Make :slow_tests tagged integration tests not run with a default rspec run.

Some tests are quite slow due to build steps. To not run all of those on local
developer machines but only on the CI server we introduce a slow_tests tag and
make execution of slow_tests optional.