hawkBit repository uses Optional on single entity find/get requests (#435)

* Repo returns optionals.
* Improved exception handling for collection usage in repo queries.

Signed-off-by: kaizimmerm <kai.zimmermann@bosch-si.com>

hawkbit-rest-core/pom.xml

@@ -62,11 +62,6 @@
          <artifactId>hawkbit-repository-test</artifactId>
          <version>${project.version}</version>
          <scope>test</scope>
-      </dependency>
-      <dependency>
-         <groupId>org.easytesting</groupId>
-         <artifactId>fest-assert</artifactId>
-         <scope>test</scope>
       </dependency>      
       <dependency>
          <groupId>ru.yandex.qatools.allure</groupId>