Update alternatives.sh to allow more alternatives

If more alternatives are set, the script will not fail.
[DOC] make links relatives and add proxy mode doc
Minor corrections on documentation
Enable keepcache in yum
Improve proxy mode documentation
Correct mistakes in proxy mode documentation
Rollback setting proxy mode as default
Correct an error in alternatives.sh
Correct a mistake in resources dir build script
Correct an error in postgresql repos task
Update nginx example vhost for proxy mode
Correct "Disable SELinux now" task

Avoid "stderr: setenforce: SELinux is disabled"
when selinux is already disabled