changed mdata-get and two initialization scripts

[MerlinDMC]

mdata-get

mdata-get tries to read from the socket even if it is not available and runs in retries until canceled manually

# ./mdata-get dummy
./mdata-get: line 49: /dev/cuau1: No such file or directory
./mdata-get: line 60: 3: Bad file descriptor
mdata-get: timeout getting status. (exit code: 142)
./mdata-get: line 60: 3: Bad file descriptor
...

I've added code to check for socket presence before the read loop starts.

set-root-authorized-keys

If no metadata is set for root_authorized_keys a false .ssh/authorized_keys file is generated.
I've added code to test if the metadata could be read before generating folders and files.

run-user-script

If the user-script is not set in customer_metadata "No metadata for user-script" is saved in /root/user-script and left there.
I've just added code to remove the file in that single case.

[MerlinDMC]

If someone has created a VM with the provided images for centos for example and forgot to add his pubkey in the metadata, changed the metadata and rebooted the VM he/she would not be able to log in because of the false authorized_keys already generated.

In that case it is enough to use VNC to login as root and delete the /root/.ssh folder and reboot again.