Restrict one instruction type per instruction

[Alopalao]

There are 7 types of instructions:

• Meter
• Apply-Actions
• Clear-Actions
• Write-Actions
• Write-Metadata
• Stat-Trigger (not supported by Open vSwitch)
• Goto-Table

There should not be duplicated intruction_type when creating a flow. For example the following should not be accepted:

            "instructions": [
            	{   
                    "instruction_type": "apply_actions", 
            		"actions": [{"port": 11, "action_type": "output"}]
                },
            	{
                    "instruction_type": "apply_actions", 
            		"actions": [{"port": 10, "action_type": "output"}]
                }
            ]