Correcting su validation and tests when su => false

I realized earlier that I had made one more change to the branch in PR #19, but forgot to push it out.  My original su changes in 4cdd03f and 2a89fde have a bug that allows su to be true, but no owner set. (The 'su' option has no 'nosu' version to go with it and would have created an invalid config.)  This PR fixes that bug.

#### Commit Message:
The su config option works differently than other logrotate config
options and does not have a corresponding 'nosu' version.  This adds
proper validation checks, tests and erb handling to allow for this.

Without this, it would be possible to have only 'su' in the logrotate
rule if no owner or group was specified.