libpod/container_internal_linux: Allow gids that aren't in the group file

When an image config sets config.User [1] to a numeric group (like
1000:1000), but those values do not exist in the container's
/etc/group, libpod is currently breaking:

  $ podman run --rm registry.svc.ci.openshift.org/ci-op-zvml7cd6/pipeline:installer --help
  error creating temporary passwd file for container 228f6e9943d6f18b93c19644e9b619ec4d459a3e0eb31680e064eeedf6473678: unable to get gid 1000 from group file: no matching entries in group file

However, the OCI spec requires converters to copy numeric uid and gid
to the runtime config verbatim [2].

With this commit, I'm frontloading the "is groupspec an integer?"
check and only bothering with lookup.GetGroup when it was not.

I've also removed a few .Mounted checks, which are originally from
00d38cb3 (podman create/run need to load information from the image,
2017-12-18, #110).  We don't need a mounted container filesystem to
translate integers.  And when the lookup code needs to fall back to
the mounted root to translate names, it can handle erroring out
internally (and looking it over, it seems to do that already).

[1]: https://github.com/opencontainers/image-spec/blame/v1.0.1/config.md#L118-L123
[2]: https://github.com/opencontainers/image-spec/blame/v1.0.1/conversion.md#L70

Signed-off-by: W. Trevor King <wking@tremily.us>

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