Investigate alternative authentication mechanisms

[Danil-Grigorev]

There are ways to reduce number of resources provisioned by agent for authentication in a Rancher cluster, by employing different authentication methods.

Investigate possible approaches.

Initial set:

• Agent can issue a CSR request, which will be approved by Rancher to approve agent authentication, with a short spec.expirationSeconds value to allow revoking access.
• Agent can connect via a single set of SA, RoleBinding and Role, using certificate and a token.