webmin: allow override of webmin login and password

Don't hardcode the webmin login and password in the install script.
Instead, extract them to variables, to allow us to override them in
a bbappend.

Signed-off-by: Anders Darander <anders@chargestorm.se>
webmin: split out webmin themes

The default set of themes taks up ~13MB, with a couple of them weighting in
at ~5MB each.

Let's split the themes to separate packages, to allow a considerable size
reduction of the core webmin package (from +15MB to 2.1MB on my build host).

Signed-off-by: Anders Darander <anders@chargestorm.se>