Improve CLI UX

[patricksanders]

This PR is focused on making the CLI more intuitive and easier to work with for users who aren't as familiar with specific AWS constructs such as IMDS. It consolidates the two distinct webserver commands (metadata and ecs_credential_helper) into a single serve command. This new command takes an optional argument to enable the IMDS endpoints for a specific role and otherwise always serves the ECS credential helper endpoints. I also added aliases to the serve command to maintain CLI compatibility with previous versions. For example, weep metadata AdminRole still works, just using the serve command instead and with the ECS credential provider running at the same time.

Additionally, short and long help texts for commands have been centralized into cmd/vars.go. This helps declutter the files for individual commands and aids in writing more consistent help text.

[castrapel]

Looking good! I encountered these issues:

1. With the vanilla flow, I would expect to need to set up my consoleme url and username if I don't already have a configuration, but it isn't prompting me:

$ ./weep serve consoleme time="2021-03-03T21:32:53-08:00" level=info msg="Configuring weep IMDS service for role consoleme" Error: Authentication method unsupported or not provided.

2. When I type ./weep, I see help in the list of commands. It doesn't indicate clearly that I need to type a command after the help arg. When I type ./weep help, I am also not told this clearly

3. When I type ./weep serve <role> (where is a string that matches multiple roles), Weep appears to try multiple times before erroring, despite getting the More than one matching role error