REST API endpoint for OAuth2 administration

Improve on the OAuth2 endpoint to allow listing, editing and deletion of OAuth 2 providers and users' tokens.

So far, the oauth2 endpoint exposes the following resources:

• GET oauth2/provider/<providerName>: to retrieve the details for a given provider
• GET oauth2/provider/<providerName>/token: to retrieve a token for the current user for a given provider

We need to add the following resources to allow cloud service administration on web ui:

• GET oauth2/provider: returns the list of registered oauth2 service providers
• POST oauth2/provider: creates a new oauth2 service provider
• PUT oauth2/provider/<providerName>: updates an oauth2 service provider
• DELETE oauth2/provider/<providerName>: deletes an oauth2 service provider
• GET oauth2/token: to list all stored tokens for all users
• GET oauth2/token/<providerName>/<nxLogin>: to retriven a stored token
• PUT oauth2/token/<providerName>/<nxLogin>: to update a stored token
• DELETE oauth2/token//<providerName>/<nxLogin>: to delete a stored token