Write the intent-to-upgrade file during upgrades
Write the intent-to-upgrade file during upgrades

To recognize that an upgrade attempt of a cluster was interrupted by a
power failure or similar, write a file into the file system documenting
that currently an upgrade is in progress.

Signed-off-by: Klaus Aehlig <4ce51de36d9068cd7db2eec1151e0f3f95419008@google.com>
Reviewed-by: Michele Tartara <05f7077a2c6cbbdc4af4ba42e3ef89a0df846179@google.com>

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