Werk #1656: cisco_vpn_tunnel: Refactored complete check, fixed threshold bugs

Improved check in different ways:

• When configuring OK as failed tunnel state, this was not handled correctly by the check.
• Clarified the valuespec titles and help texts.
• Check did not process multiple VPN tunnel configurations defined in a single rule.
• Cleaned up check code

Sadly had no data for testing here... Let's hope for the best.

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