Add os_name alias mechanism

[jeremyschulman]

In order to avoid copy/paste of os_name section content, provide a means to alias one on_name to another. For example alias "catos" to be the same as "ios":

[os_name.ios]
    aliases = ['catos']
    pre_get_config = "terminal length 0"

[jeremyschulman]

Adding deferred since I do not really need this at this time; but if there is enough interest I should be able to do this fairly quickly.