lxc-start-ephemeral: Parse passwd directly

lxc-start-ephemeral: Parse passwd directly

On Ubuntu 15.04, lxc-start-ephemeral's call to pwd.getpwnam always
fails.  While I haven't been able to prove it or track down an exact
cause, I strongly suspect that glibc does not guarantee that you can
call NSS functions after a context switch without re-execing.  (Running
"id root" in a subprocess from the same point works fine.)

It's safer to use getent to extract the relevant line from the passwd
file and parse it directly.

Signed-off-by: Colin Watson <cjwatson@ubuntu.com>