Correct test for existance of instances
Correct test for existance of instances

Since python interprets an empty dictionary as 'False',
testing for the existance of the 'instances' field in
the configuration has to be done by explicitely testing
for 'None'.

Signed-off-by: Helga Velroyen <2d0126b1ec6f9ec5cbec4a7012aa14faeaef10f5@google.com>
Reviewed-by: Klaus Aehlig <4ce51de36d9068cd7db2eec1151e0f3f95419008@google.com>

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