ovs-bugtool: Change the default output type.

ovs-bugtool: Change the default output type.

In my tests, I see that creating a tar.gz takes a lot less
time than creating a tar.bz2. The difference in compressed
size of the debug bundle is not much different when either
of the above is used. So, use tar.gz as the default debug
bundle type.

Test results in my setup:
For an uncompressed debug bundle size of 250MB(95% of it is log files),
bz2 takes 50 seconds whereas gz takes 8 seconds. xz took 90 seconds.
gz, bz2 and xz compressed the debug bundle into 144M, 139M and 131M
respectively.

Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>