'machine' authentication to impersonate users through a shared secret

[gmgigi96]

This PR adds a new authentication provider, called machine, in order to impersonate
an user through a shared secret, stored in the reva configuration.

Example:

$ reva -insecure -host localhost:19000 login  -username einstein -api-key qwerty123 machine
OK
$ reva -insecure -host localhost:19000 whoami
id:<idp:"localhost:20080" opaque_id:"4c510ada-c86b-4815-8820-42cdf82c3d51" type:USER_TYPE_PRIMARY > username:"einstein" mail:"einstein@example.org" display_name:"Albert Einstein" groups:"sailing-lovers" groups:"violin-haters" groups:"physics-lovers" 
$ reva -insecure -host localhost:19000 login  -username marie -api-key qwerty123 machine
OK
$ reva -insecure -host localhost:19000 whoami
id:<idp:"localhost:20080" opaque_id:"f7fbf8c8-139b-4376-b307-cf0a8c2d0d9c" type:USER_TYPE_PRIMARY > username:"marie" mail:"marie@example.org" display_name:"Marie Curie" groups:"radium-lovers" groups:"polonium-lovers" groups:"physics-lovers"

[labkode]

@gmgigi96 easy one to fix