Allow passing any uid for container processes.
Allow passing any uid for container processes.

Usually if a username is provided when starting a process inside the container
we look inside the /etc/passwd file of the container to find the uid and gid for that
user. However, if a uid is provided instead of a username there is no need to look into
the /etc/passwd file to see if that user exists.

Signed-off-by: Amit Barve <9f8690dd9086378e9a909428c98b6de14f7fdc30@microsoft.com>

rn/opengcs,rn/opengcs,Microsoft/hcsshim