Fix batch creation of instances
Fix batch creation of instances

During batch creation of instances the 'reason' field in opcodes is set
to None (but the field is present). This caused problems when adding a
reasons to the reason trail.

Setting the default value for the 'reason' field to the empty list fixes
this issue.

Signed-off-by: Thomas Thrainer <98bdeac947eb8ca18146e9ce7f092788ec242cd1@google.com>
Reviewed-by: Michele Tartara <05f7077a2c6cbbdc4af4ba42e3ef89a0df846179@google.com>

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