Enable homedir creation

We should enable automatic home directory creation in realmd, when it is used to join a domain. Otherwise, all our instructions have to be followed by "And please call `sudo pam-auth-update --enable mkhomedir`".

realmd has a series of service/realmd-<distro>.conf files, one per distro it supports. Ubuntu uses the debian one. Inside that file, there is a [commands] section. On RH, for example, it's:
[commands]
winbind-enable-logins = /usr/bin/sh -c "/usr/sbin/authconfig --update --enablewinbind --enablewinbindauth --enablemkhomedir --nostart && /usr/bin/systemctl enable oddjobd.service && /usr/bin/systemctl start oddjobd.service"
...

Notice how it's there that they enable home dir creation, via the oddjob package/service. For debian, we have:
[commands]

# Various pam and nss plugins are automatically configured when
# installed on debian. So just stub them out.

winbind-enable-logins =
winbind-disable-logins =
sssd-enable-logins =
sssd-disable-logins =
...

I suggest we change service/realmd-debian.conf to include:

[commands]
winbind-enable-logins = /usr/sbin/pam-auth-update --package --enable mkhomedir
winbind-disable-logins = /usr/sbin/pam-auth-update --package --remove mkhomedir
sssd-enable-logins = /usr/sbin/pam-auth-update --package --enable mkhomedir
sssd-disable-logins = /usr/sbin/pam-auth-update --package --remove mkhomedir

As the other bits about nss and pam are already taken care by the package's themselves in postinst, like the comment above says about the debian case.