Add support for contexts

[maxhoesel]

Recent versions of step-cli support the use of contexts to enable switching between multiple CAs:

https://smallstep.com/docs/step-cli/the-step-command/#contexts-working-with-multiple-cas

It would be great if our collection could support this feature, both for modules and roles.

For modules, this would mean adding the --context parameter to override the context,
while for the roles, this would mean adding support for bootstrapping into a specific context, and then using it from other roles.

Tracking issues:

• Implement context param for modules
• Allow setting context in step_bootstrap_host and step_acme_cert