Use Enums instead of String for sourceType and destinationType in block mappings

[gschukin]

Currently BlockDeviceMappingBuilder#sourceType and BlockDeviceMappingBuilder#destinationType use string as parameter. According to documentation
sourceType can be one of:

• image
• volume
• snapshot
• blank

destinationType can be one of:

• local
• volume

[gondor]

Now using Enums