Add update group command

[dadgar]

Changes proposed in this PR:

Add hcp iam groups update command.

How I've tested this PR:

$ hcp iam groups create cli-test --description "cli-test"
Resource Name: iam/organization/11eb5071-5393-1932-a04c-0242ac110014/group/cli-test
Resource ID:   iam.group:gJBqc6FHfzDJhHczPNTc
Display Name:  cli-test
Member Count:  0
Description:   cli-test

$ hcp iam groups update cli-test
ERROR: either display name or description must be specified

$ hcp iam groups update cli-test --display-name "updated-name" --description "updated test"
✓ Group "iam/organization/11eb5071-5393-1932-a04c-0242ac110014/group/cli-test" updated

$ hcp iam groups read cli-test
Resource Name: iam/organization/11eb5071-5393-1932-a04c-0242ac110014/group/cli-test
Resource ID:   iam.group:gJBqc6FHfzDJhHczPNTc
Display Name:  updated-name
Member Count:  0
Description:   updated test

$ hcp iam groups delete cli-test
The group will be deleted.

Do you want to continue (y/n)? y

✓ Group "iam/organization/11eb5071-5393-1932-a04c-0242ac110014/group/cli-test" deleted

How I expect reviewers to test this PR:

Check the update command covers all updatable fields, and has good documentation.

Checklist:

• Tests added if applicable
• CHANGELOG entry added or label 'pr/no-changelog' added to PR

  Run make changelog-entry for guidance in authoring a changelog entry, and
  commit the resulting file, which should have a name matching your PR number.
  Entries should use imperative present tense (e.g. Add support for...)

[squaresurf]

Great work on this! 🙌