docs: Upgrade instructions

docs: Upgrade instructions

New section specific to upgrade was added into documentation.
Description of configuration changes beetween Colorado and
Danube releases is discussed in detail.

JIRA: VSPERF-269

Change-Id: Ibbd1bb447e5d85c6b269ad26e10ff5d646a2bfbe
Signed-off-by: Martin Klozik <martinx.klozik@intel.com>
Reviewed-by: Al Morton <acmorton@att.com>
Reviewed-by: Christian Trautman <ctrautma@redhat.com>
Reviewed-by: Bill Michalowski <bmichalo@redhat.com>
Reviewed-by: Antonio Fischetti <antonio.fischetti@intel.com>
Reviewed-by: Sridhar Rao <sridhar.rao@spirent.com>
Reviewed-by: Martin Goldammer <martinx.goldammer@intel.com>