Enable Oauth2 for most http-based notifiers

[JacobsonMT]

Followup to #338

This PR contains two commits:

• Move http_config to a common area: This moves http_config into a more central area so it can be easily extended to other notifiers and their tests. http_config is now un-marshalled separate from the notifier-specific configuration.
• Enable Oauth2 for most http-based notifiers: This does the actual enabling on the common HTTP config options to most other notifiers. A much simpler PR because of the above prep.

Testing of the common HTTP config options (currently just oauth2) is fairly comprehensive and centralized in notify/receivers_test.go instead of spread out among each notifier's package tests. The tests use mock servers to ensure the token is actually being correctly retrieved from the oauth2 server and the Authorization header set on outgoing http calls.

Notifiers not yet supported and why:

• Email: Choice of implementation isn't clear.
• Slack: Needs to use standard http client and handle Bot Token required fields if oauth2 is configured without a token.
• SNS: Needs to integrate with standard http client.
• MQTT: Implementation non-trivial.
• Alertmanager: Needs to use standard http client.

[yuri-tceretian]

LGTM
I am not a fan of the making the model generic for all integrations as there might be situations when particular implementation would require different settings and maintaining aggregated model would make it a nightmare. However, I understand the reason why you do it now, and I think it makes sense.