Strip defaults from namespace flags

We were picking up defaults from the client's containers.conf,
which broke pod namespaces. The server-side code already checks
containers.conf when not explicitly overridden by the user, or by
the container being part of a pod (the last bit being our bug).

This only manifested on systems with a containers.conf installed,
so RHEL 8 and Fedora 32 (which means our F32 CI VMs likely should
have caught it, but didn't, which is concerning).

This prevents defaults for these flags being shown, but they were
incorrect anyways for `podman-remote`, so I'm not terribly
concerned.

Signed-off-by: Matthew Heon <matthew.heon@pm.me>

cmd/podman/common/create.go

@@ -50,7 +50,7 @@ func GetCreateFlags(cf *ContainerCLIOpts) *pflag.FlagSet {
 		"Drop capabilities from the container",
 	)
 	createFlags.String(
-		"cgroupns", containerConfig.CgroupNS(),
+		"cgroupns", "",
 		"cgroup namespace to use",
 	)
 	createFlags.StringVar(
@@ -244,7 +244,7 @@ func GetCreateFlags(cf *ContainerCLIOpts) *pflag.FlagSet {
 		"Keep STDIN open even if not attached",
 	)
 	createFlags.String(
-		"ipc", containerConfig.IPCNS(),
+		"ipc", "",
 		"IPC namespace to use",
 	)
 	createFlags.StringVar(
@@ -325,7 +325,7 @@ func GetCreateFlags(cf *ContainerCLIOpts) *pflag.FlagSet {
 	)
 	// markFlagHidden(createFlags, "override-os")
 	createFlags.String(
-		"pid", containerConfig.PidNS(),
+		"pid", "",
 		"PID namespace to use",
 	)
 	createFlags.Int64Var(
@@ -454,11 +454,11 @@ func GetCreateFlags(cf *ContainerCLIOpts) *pflag.FlagSet {
 		"Username or UID (format: <name|uid>[:<group|gid>])",
 	)
 	createFlags.String(
-		"userns", containerConfig.Containers.UserNS,
+		"userns", "",
 		"User namespace to use",
 	)
 	createFlags.String(
-		"uts", containerConfig.Containers.UTSNS,
+		"uts", "",
 		"UTS namespace to use",
 	)
 	createFlags.StringArrayVar(