Add validation of manifest for helm packages.

Add validation of manifest for helm packages.

For ONAP native zip packages added validation of manifest file.
HELM package is only valid if:
 * isBase flag is set to 'true' for exactly one helm entry
 * isBase flag is present for all helm entries
Zip package is considered to be a helm package if it contains HELM entries but does not contain any HEAT entries (however it may contains entries of other types).

Refactored method OnboardingPackageProcessor.processPackage():
 * simplified logic
 * enhanced exception handling

Issue-ID: SDC-3185
Signed-off-by: Jan Malkiewicz <jan.malkiewicz@nokia.com>
Change-Id: Ica3b1f1504ce4fc3a671c4b8fa8de2bf0236bd77