Add Prometheus bearer token authentication

[n35k3nd1r]

Adds support for authenticating against Prometheus with a bearer token or token file.

This keeps the existing basic auth support, but makes basic auth and bearer token auth mutually exclusive.
The daemon can now read prometheus.token or prometheus.token_file, persist locked Prometheus config values, and use them for Prometheus API requests. The web module adds the matching configuration fields.

Feature Request: #211

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[n35k3nd1r]

Related pull request Icinga/icinga-kubernetes-web#177

[n35k3nd1r]

CLA signed

[bobapple]

@cla-bot check

[n35k3nd1r]

@lippserd Could you take a look when you have time?