[NEXUS-5901] Warn user about the fact that security has been enabled on upgrade

Signed-off-by: Alin Dreghiciu <adreghiciu@gmail.com>
[NEXUS-5903] Remove config/api for disabling security

Signed-off-by: Alin Dreghiciu <adreghiciu@gmail.com>
[NEXUS-5902] Use a shire thread bounded all rights subject in case that tests should run with security disabled

+ remove direct calls to security system/config to disable security

Signed-off-by: Alin Dreghiciu <adreghiciu@gmail.com>
[NEXUS-5899] Do not store the upgraded file to ${plexus-home}/conf as that may interfere with other tests depending on test ordering

Signed-off-by: Alin Dreghiciu <adreghiciu@gmail.com>