b-515: Corrects vlan_id prevalence in vnet creation

[aleixrm]

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Description

Some suggestions for #561

• Corrects vlan_id and automatic_vlan_id precedence, and refactors conditional.
• Adds some tests for checking conflict between vlan_id and automatic_vlan_id and the fixed bug.
• Some formatting

References

#561
#515

New or Affected Resource(s)

• opennebula_virtual_network

Checklist

• I have created an issue and I have mentioned it in References
• My code follows the style guidelines of this project (use go fmt)
• My changes generate no new warnings or errors
• I have updated the unit tests and they pass succesfuly
• I have commented my code, particularly in hard-to-understand areas
• I have updated the documentation (if needed)
• I have updated the changelog file