When a Nomad agent starts and loads jobs that already existed in the
cluster, the default template uid and gid was being set to 0, since this
is the zero value for int. This caused these jobs to fail in
environments where it was not possible to use 0, such as in Windows
clients.
In order to differentiate between an explicit 0 and a template where
these properties were not set we need to use a pointer.
UID/GID 0 is usually reserved for the root user/group. While Nomad
clients are expected to run as root it may not always be the case.
Setting these values as -1 if not defined will fallback to the pervious
behaviour of not attempting to set file ownership and use whatever
UID/GID the Nomad agent is running as. It will also keep backwards
compatibility, which is specially important for platforms where this
feature is not supported, like Windows.
