gnt-cluster modify: verify instance's disk template usage
gnt-cluster modify: verify instance's disk template usage

Adds a verification step to 'gnt-cluster modify' to make sure that no
disk template is disabled that is currently in use by at least one instance.

Signed-off-by: Helga Velroyen <2d0126b1ec6f9ec5cbec4a7012aa14faeaef10f5@google.com>
Reviewed-by: Michele Tartara <e28994c840ce8e1c5c7d881013b818e2358d00c1@google.com>

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