Fix OMERO.cpp integration tests

With the recent SV1+SV2 fix, users with empty passwords are disabled by
default. This commit fixes the OMERO C++ integration tests by using
createExperimentWithPassword to create users with a password equal to the
username and fixing the tests login().
Fix syntax of experimenter creation with password
Fix PermissionsTest.testAdjustPermissions login()
Create client without session to retrieve omero.rootpass
Fix user creation in CmdCallback fixture
Fix omero api exception in cpp sessions test
Merge pull request #3074 from sbesson/omerocpp_login_fix

Fix OMERO.cpp integration tests