Make --tag optional during bootstrap command; raise errors as needed

[apurvis]

https://github.com/lumoslabs/broadside/pull/80 requires the --tag every time bootstrap is run.

inside bootstrap though, the only part that requires the tag option is the creation of the task_definition. once the task defintion is up, the service can continue being setup without the option.

so.... i'm pretty unsure if @slpsys 's solution over on https://github.com/lumoslabs/broadside/pull/80 is overkill (because --tag isn't always needed, depending on the state of your services and task_definitions), or if this PR is underkill in that it lets you get away with doing stuff with less typing in more obscure situations (by "obscure" i mean only very rarely do people find themselves with a well formatted task_defintion and no service definition at all)

(maybe the long termer option is to have bootstrap tear down the existing service and start over.)

[apurvis]

i actually think this PR was a little bit incorrect - no matter what you need a tag to create the first task_definition. you only don't need it when you're creating a service.

so i pared this back a bit; think this is the least invasive way to raise errors on bad inputs (which is almost no code change, honestly)

[slpsys]

🆒