Add tokenrealm for yubikey_mass_enroll

[plettich]

When yubikey_mass_enroll is used, the tokens won't have a default tokenrealm assigned. We should add a possibility to add the to a specific realm during enrollment.

[cornelinux]

Sounds nice and sensible. Like low hanging fruit.
Currently this could be achieved using an event handler.

A command could look like this:

privacyidea -U https://my.pi --admin super token yubikey_mass_enroll --realm newrealm

Internally during /token/init the parameter realm could be added.