Add two new cluster settings
Add two new cluster settings

The new variables are:

- a list of hidden OSes, that should not be displayed to the users in
  interactive selection (e.g. reinstall); however, if they are selected, they
  can be used
- a list of OSes that should be hidden and blocked from install-time selection

The filtering will apply at pure OS name level, not OS+variant level.

Signed-off-by: Iustin Pop <acf83a5274f31a43af6b914614365999469c1a31@google.com>
Reviewed-by: Guido Trotter <f62ca4d5358bdaeef0e50e57f9f7e231de63727e@google.com>

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