Speedup function cobbler.utils.is_mac

Use both lower case and upper case hexadecimal digits in the regex
instead of using the re.IGNORECASE option. Also add more tests.
timeit with 64M iterations: 42.78 versus 39.74 seconds.
Merge pull request #413 from ciupicri/master-mac-regex

Speedup function cobbler.utils.is_mac