Make agent support new endpoint for gathering data

[j-fuentes]

This adds support for the new form of the endpoint that gathers data.

It introduces significant changes in the configuration, but everything should be backward compatible.

These changes are:

• The Schedule field is not optional, as we are using the period flag at the moment.
• The Token string is also optional, as it should not be specified when using OAuth2.
• The Endpoint is being deprecated, as in the future you won't configure the endpoint for uploading data, just preflight server. At the moment it is still being used if specified, but ignored and defaulting to /api/v1/datareadings otherwise.
• Added a new Server property that points to the Preflight server. Defaults to https://preflight.jetstack.io if not specified.
• Added OrganizationID which will be used with the new endpoint.
• Added ClusterID. To be left empty if using agent tokens (as they already point to a cluster).

Signed-off-by: Jose Fuentes jsfuentescastillo@gmail.com

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[charlieegan3]

/lgtm