Removed maven assembly plugin version from stratos parent pom.xml to be inherited from apache pom.xml

Signed-off-by: Imesh Gunaratne <imesh@apache.org>
Updated pom.xml and bin.xml files to fix assembly id being mandatory after updating maven-assembly-plugin version to 2.2.1

Signed-off-by: Imesh Gunaratne <imesh@apache.org>