Incorrect global flags for snapctl

[geauxvirtual]

The help for snapctl lists global flags for url and apiversion as having two options:
--url or -u and --api-version or -a. However, neither -u or -a actually work because the code in cmd/snapctl/main.go is checking for --u and --a. Single letter options should only need a single dash in front of them.

https://github.com/intelsdi-x/snap/blob/master/cmd/snapctl/main.go#L67-L78

It could be argued that passing --a or --u should throw an error for an invalid flag instead of just ignoring the flag.