add user agent to kube-api calls

[aramase]

Reason for Change:

Add user-agent to kube-api calls.

Issue Fixed:

Fixes #352

Notes for Reviewers:
Sample output from grafana -

{client="aad-pod-identity/MIC/0.0.0-dev/6f597c3/2019-08-21-06:53",resource="azureassignedidentities",verb="CREATE"}
{client="aad-pod-identity/MIC/0.0.0-dev/6f597c3/2019-08-21-06:53",resource="azureidentities",verb="LIST"}
{client="aad-pod-identity/NMI/0.0.0-dev/6f597c3/2019-08-21-06:52",resource="azureassignedidentities",verb="LIST"}