Disable compression for all intra-cluster imports/exports

Tests have shown that usually we're CPU-bound for intra-cluster
imports/exports. Disabling compression will help with this.

Some versions of OpenSSL, depending on the build options, also
compress transparently. This will need further work in Ganeti.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>