`create-policy` should warn that it cannot create tags

When the user runs an outdated `create-policy` command that was intended to
create a tag, the user sees the unhelpful: `error: tags[0] should be a string,
but was actually a object`. This should instead read: `error: this command can
no longer create tags; see `razor help create-tag``.

Fixes https://tickets.puppetlabs.com/browse/RAZOR-336
Standardize aliases in commands

Aliases are becoming a common design pattern. This commit refactors those into
the Command class.

For https://tickets.puppetlabs.com/browse/RAZOR-276
Alias 'c' for 'configuration' in `create-broker`

To abbreviate the `create-broker` command, this adds "c" as an alias. In
the process of doing that, this commit refactors the behavior of creating
aliases into the Command class.

Fixes https://tickets.puppetlabs.com/browse/RAZOR-276