Sonar issue resolutions for RestClientInvoker.java

    Resolve Blocker issue:

    https://sonar.onap.org/issues/search#issues=AWBzdRXQ-08if2a6vsl1

    And other sonar findings:

    https://sonar.onap.org/issues/search#issues=AWBzdRXP-08if2a6vslw
    https://sonar.onap.org/issues/search#issues=AWBzdRXP-08if2a6vslx
    https://sonar.onap.org/issues/search#issues=AWBzdRXP-08if2a6vsly
    https://sonar.onap.org/issues/search#issues=AWBzdRXQ-08if2a6vslz
    https://sonar.onap.org/issues/search#issues=AWBzdRXQ-08if2a6vsl0
    https://sonar.onap.org/issues/search#issues=AWBzdRXP-08if2a6vslv
    https://sonar.onap.org/issues/search#issues=AWBzdRXQ-08if2a6vsl2

Gerrit Reviews:

https://gerrit.onap.org/r/#/c/27899/