Adopt dedicated 'rockstor' user for systemd services

[phillxnet]

In line with reviewing our general good practice, it is proposed that we establish a dedicated system user, with specifically limited rights, and instruct our systemd services to use this user where-ever possible to run our various services. We do, via our systemd initiated services, manage many privileged areas of the system, and so will likely have to award certain capabilities to our services/user: but along with the following issues:

• Avoid system calls to interact with systemd services Avoid system calls to interact with systemd services #2680
• Establish rockstor-apparmor Establish rockstor-apparmor.service #2699

These escalated privileges can be explicitly formalised and constrained to exactly what is required and no more.