Replace submission-api --api-host cli option with --amo-base-url

[rpl]

As a followup to the original discussion in #2458 about the --api-host option.

@willdurand @Rob--W and I have discussed about it one more time during today's issue triage and the following is a summary of the changes we agreed on:

• rename the option from --api-host to --amo-base-url
• the default value for --amo-base-url will be set to "https://addons.mozilla.org/api/v5"

This option is meant to mainly be used by QA to run QA verifications on the staging or dev AMO servers, and it is reasonable to most extension developers will not need to use it or to customize the url, and so we agreed that we may not need to apply any special check or restrictions to the value got from it (well besides being a valid url that doesn't throw when passed to new URL(...)), we may also consider throwing or ignoring any first that isn't going to be needed or used (e.g. only pick up scheme, origin and path and ignore the rest or throw is not empty).

[willdurand]

the default value for --amo-base-url will be set to "addons.mozilla.org/api/v5"

https://addons.mozilla.org/api/v5, right? (maybe with the trailing slash even but at least with the scheme)