Fix get_metadata name of AuthInfo class (#2221)

[PhilippRue]

Fixes #2221
Fixes this error:

Error: * The test raised an exception!
** Full traceback:
   Traceback (most recent call last):
     File ".../aiida_core/aiida/cmdline/commands/cmd_computer.py", line 561, in computer_test
       succeeded = test(transport=trans, scheduler=sched, authinfo=authinfo)
     File ".../aiida_core/aiida/cmdline/commands/cmd_computer.py", line 145, in _computer_create_temp_file
       workdir = authinfo.get_workdir().format(username=remote_user)
     File ".../aiida_core/aiida/orm/authinfos.py", line 136, in get_workdir
       metadata = self.get_metadata()
     File ".../aiida_core/aiida/orm/authinfos.py", line 122, in get_metadata
       return self._backend_entity.get_metadata()
   AttributeError: 'DjangoAuthInfo' object has no attribute 'get_metadata'
Some tests failed! (1 out of 4 failed)

when running verdi computer test ...

[coveralls]

Coverage increased (+0.09%) to 68.887% when pulling ad20750 on JuDFTteam:develop into 60bc398 on aiidateam:develop.

[muhrin]

Make the linter happy

[muhrin]

Suggested change

	return self._backend_entity._get_metadata()
	return self._backend_entity._get_metadata() # pylint: disable=protected-access

[muhrin]

All good. A more general solution will come in when provenance_redesign gets merged. Thanks @PhilippRue