Do not change process UID

When run with sudo, Anvil changed process real UID
to SUDO_UID thus easily creating files with different owners.
This mechanism can confuse some tools (like pip)
Now, we can almost safely stop changing real UID and run
prepare as non-root and bootstrap, install, start, status,
and stop as root.

Also, this removes dependency on sudo.

Implements: blueprint no-uid-tricks
Fixes: bug #1179747
Fixes: bug #1186440
Fixes: bug #1186448

Change-Id: I7ae293aad7f0a5ba08962e6b6b709fe49b8b81ec