Improve upgrade script. The nut and ldap plugins must be deinstalled before upgrade, otherwise the installation fails.

git-svn-id: svn://svn.code.sf.net/p/openmediavault/code@1460 3c3d7b80-5828-415e-9f61-280b98178fff
Remove iSCSI target plugin before release upgrade to prevent broken upgrades.

git-svn-id: svn://svn.code.sf.net/p/openmediavault/code@1461 3c3d7b80-5828-415e-9f61-280b98178fff
Improve upgrade script. Fix issue when upgrading from squeeze-lts.

git-svn-id: svn://svn.code.sf.net/p/openmediavault/code@1462 3c3d7b80-5828-415e-9f61-280b98178fff
Update changelog

git-svn-id: svn://svn.code.sf.net/p/openmediavault/code@1463 3c3d7b80-5828-415e-9f61-280b98178fff