Use /srv/prometheus instead of /data, because user `prometheus` doens't
have permissions to create `/data/` in `/`.

Instead this commit puts prometheus data into `/srv/prometheus` by
default, which is a cleaner path, and it'll create tsdb path & then assign
required permissions on each prometheus service start.

This way, also, the cases when users re-configure tsdb to point to
external USB - it'll also be created and assigned required permissions for
prometheus.

Signed-off-by: Alex Simkin <sashasimkin@gmail.com>