Add OCUnlink / OCRemove API to Provisioning Manager

Add OCUnlink / OCRemove API to Provisioning Manager

- Since security admin should revoke disclosed credential,
  these API give capability to security admin to manage
  symmetric key type credentials in his/her network.

[Patch #3,#4] Fix wrong behavior of functions, Correct style, unittest

Change-Id: I73f14c207511ce85d739da0504aa8c65259b11cf
Signed-off-by: Woochul Shim <woochul.shim@samsung.com>
Signed-off-by: Randeep Singh <randeep.s@samsung.com>
Signed-off-by: Woochul Shim <woochul.shim@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/2407
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Sachin Agrawal <sachin.agrawal@intel.com>