install: adding some notes to triggering system upgrade by packages installation.
install: rewriting checking of GISLAB_INSTALLATION_DONE to one line.
install: better handling of connection forwarding script.
install: do not overwrite '/etc/fstab' file on NFS installation.
install: set higher value of system shmmax to run PostgreSQL properly.
install: less use 'sed' command, rather write whole configuration file if short.
install: do not set custom hostname for cloud providers.