Refinements to RestApiCall plugin

Changes includes:
1) Check for null in JsonParser.convertToProperties() which can otherwise result in null pointer exception
2) Use logger built-in string formatting rather then string concatenation
3) Use StringBuilder for multiple string concatenations
4) Making utility classes final and defines private constructor for them
5) Added testcases/testpoints
https://sonar.onap.org/component_issues/index?id=org.onap.ccsdk.sli.plugins%3Accsdk-sli-plugins#resolved=false|severities=CRITICAL

Change-Id: Ic047b6d0369827a38a98c52e8365f1fe7266840f
Issue-Id: CCSDK-67
Signed-off-by: Gaurav Agrawal <gaurav.agrawal@huawei.com>