Add support for keycloak idm

A first pass at adding external keycloak account support in ohmage.

What you get:

parsing and validating of JWT tokens matching the config passed in keycloak.json (add this to ohmage.conf as keycloak.config=/path/to/keycloak.json
An internal representation of the keycloak user will be created on first login if one doesn't exist.
on a server with keycloak_auth_enabled set to false, all client/server interactions should maintain their existing functionality.
...and of course, by way of keycloak, authentication from: google, facebook, github, ldap, SAML(shibboleth) and a number of other oauth2 providers.

What you don't get:

calls that would normally update a user's personal info or password will return 500 when you attempt with an external user.
personal info for an external user is not updated when it changes on the keycloak server
A new local_auth_enabled parameter has also been created but currently does nothing.

ohmage / server