Represent memory unit in string

Enable unit representation for property 'mem_size' in
Compute node.
1. valid units are: MB or GB, MB being default choice
2. allow space separator(s) between number and unit
3. no space between number and unit is also allowed
4. use default value if property mem_size is not given
5. default value is 1024 MB

implements blueprint: compute-type-memory-unit-wd2

Change-Id: I124524200b6bde7f32ceafd0d45d412026f4ee1f
Merge "Represent memory unit in string"