NetTestSlave: use the Common.Config.DefaultRPCPort constant

For some reason the DefaultRPCPort constant was defined in two places.
This commit fixes that and removes the constant definition from the
NetTestSlave module. Instead the constant is imported from the
Common.Config module.

Signed-off-by: Ondrej Lichtner <olichtne@redhat.com>
Signed-off-by: Jiri Pirko <jiri@resnulli.us>
ExecCmd: exec_cmd processes should close fds

When creating a subprocess using the subprocess.Popen method, the newly
created process inherits the file descriptors of the parent by default.
This includes the server socket on the Slave machines which can cause
problems if the slave starts a daemon process (for example ovs-vswitchd)
that is not stopped when lnst-slave dies.

This commit fixes that.

Signed-off-by: Ondrej Lichtner <olichtne@redhat.com>
Signed-off-by: Jiri Pirko <jiri@resnulli.us>