pb-sos: effectively compress the pb-sos file with gzip

pb-sos: effectively compress the pb-sos file with gzip

Currently the pb-sos tool creates a TAR file with logs, but without
compressing it using gzip, for example. Even the output of command
says "Compressing...", but in fact no compression is done.

This patch uses gzip to effectively compress the logs. It achieves
83% of compression, observed after a simple experiment.
Also, makes use of $tarflags variable instead of pass the flags
directly in the command call.

Signed-off-by: Guilherme G. Piccoli <gpiccoli@linux.vnet.ibm.com>
Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>