Set securityContext for haproxy init container

[phihos]

The init container is currently the only container where no securityContext could be set. That is causing issues in environments with strict security constraints. This commit introduces a new key controller.haproxy.initContainer.securityContext similar to existing securityContext keys.

[jcmoraisjr]

Indeed, and thanks for the PR! I'm wondering however if it fits under controller.initContainer.securityContext instead, since .haproxy is dedicated for the haproxy container sidecar. Moreover, please add a line in the readme file.