Add "mode" option to the st2auth service.
Throw / abort if a certificate or key file doesn't exist.
Always resolve to a full path.
Add "--secure" option to the auth api service and make HTTPS optional. When
running in "proxy" mode behind Apache / Nginx, most users will probably want
Apache / Nginx to terminate SSL.
Add passlib st2auth dependency.
Add support for "standalone" auth to the st2api.

Include initial support for a httpasswd file auth backend.
Return error message on auth failure.
Invert the logic to less error prone one - default to access denied.
Add tests for file auth backend.
Return full URL to the API endpoint in the "X-API-URL" header of successful
auth response.
Fix a typo.
Merge branch 'master' into st2auth_standalone_support
Allow all the st2auth options to also be specified via the command line
arguments.
Add description for all the st2auth options.
Add MongoDB authentication backend.
Add tests for MongoDB auth backend.
Dynamically import the module so the dependencies for all the backends don't
need to be installed.
Better error handling.
Fix lint.
Update affected tests.