documentation: Installation prerequisites

documentation: Installation prerequisites

Added a mention of the default Python3 location used by virtualenv
creation script after Python3 enforcement patch(/usr/bin/python3).
Also added a paragraph warning of held-back kernel package version and
DNF/APT/YUM configuration preventing modification of kernel packages.

JIRA: MSP-10754

Change-Id: I1ba630cf85af4d99cc2cbed40163a9137b95c390
Signed-off-by: Richard Elias <richardx.elias@intel.com>
Reviewed-by: Martin Klozik <martinx.klozik@intel.com>
Reviewed-by: Al Morton <acmorton@att.com>
Reviewed-by: Christian Trautman <ctrautma@redhat.com>
Reviewed-by: Sridhar Rao <sridhar.rao@spirent.com>
Reviewed-by: Trevor Cooper <trevor.cooper@intel.com>
Reviewed-by: Ian Stokes <ian.stokes@intel.com>