spec: uid/gid for the eventHandler

[ibukanov]

The spec should clarify that eventHandler uses the same user, group and working directory to run the exec command as the main exec.

It would be also nice to have an option to specify own user and group for eventHandler. This would allow, for example, for the pre-start handler to run as a root to prepare a directory structure and fix ownership in the mounted volume before running the main application as a non-root user.

[jonboulle]

Closing as dupe of #58. Looks like that one stalled out on waiting for a PR proposal, feel free to submit one to address this. Thanks!