Make secrethub_generate idempotent

[SimonBarendse]

Change request

Current behavior

When running a playbook with a secrethub_generate several times,
each time a new version of the secret is written.

Expected behavior

When running a playbook with a secrethub_generate several times,
only the first time a new version of the secret is written.

Proposal

When the secret already exists, no new secret is generated.

To still be able to rotate existing secrets, we add a boolean
rotate parameter with a default of false. When rotate is set, a
new version of the secret will be generated when the secret already
exists (current behavior).

The module will reflect in its return whether it has changed the
secret value.