Make parameter os_dir mandatory for _OSOndiskversion.
Make parameter os_dir mandatory for _OSOndiskversion.

First part of the OS search path cleanup. _OSOndiskversion is only ever called
once, and with that argument set, so let's make it mandatory.

Reviewed-by: iustinp



dimara/ganeti,yiannist/ganeti,leshchevds/ganeti,grnet/snf-ganeti,onponomarev/ganeti,yiannist/ganeti,andir/ganeti,leshchevds/ganeti,ganeti/ganeti,onponomarev/ganeti,ganeti-github-testing/ganeti-test-1,ganeti/ganeti,dimara/ganeti,mbakke/ganeti,yiannist/ganeti,andir/ganeti,apyrgio/ganeti,apyrgio/ganeti,leshchevds/ganeti,ganeti/ganeti,mbakke/ganeti,ganeti-github-testing/ganeti-test-1,andir/ganeti,grnet/snf-ganeti,apyrgio/snf-ganeti,bitemyapp/ganeti,apyrgio/snf-ganeti,bitemyapp/ganeti,mbakke/ganeti