Use exponential backoff when fetching user-data from an URL.

The user-cloudinit-proc-cmdline systemd unit is responsible for
fetching user-data from various sources during the cloud-init
process. When fetching user-data from an URL datasource we face
a race condition since the network may not be available, which
can cause the job to fail and no further attempts to fetch the
user-data are made.

Eliminate the race condition when fetching user-data from an URL
datasource. Retry the fetch using an exponential backoff until
the user-data is retrieved.

Fixes issue 105.