feat(credential): generic OAuth2 support

[zregvart]

Given enough parameters OAuth2 support in credentials can be generic enough, this adds support for using OAuth2 provider for any connector that declares that it supports it by tagging a property with authentication-type tag and having set that property value to oauth2.

Other properties that are used also need to be tagged:

• oauth-access-token - property that will hold the value of the OAuth token
• oauth-access-token-url - URL for the auth code to token exchange
• oauth-authentication-url - URL for the OAuth authentication endpoint
• oauth-authorization-url - URL for the OAuth authorization endpoint
• oauth-authorize-using-parameters - toggle true/false if using OAuth authorization via query parameters instead of basic-auth header
• oauth-token-strategy - used with RestTemplate (not used by Syndesis currently)

For details about this options consult Spring Social project.

Fixes #727

[jimmibot]

@zregvart, thanks! @KurtStam, please review this.

[zregvart]

Moved to https://github.com/syndesisio/syndesis/pull/396