ubuntu-server-cloudimg-modify: fix location of #! shell line

ubuntu-server-cloudimg-modify: fix location of #! shell line

The #!/bin/bash was not on the first line of the script so the script
was run as sh not bash. This meant some obvious things like ${VAR} do
not work.

Change-Id: Ieac3e4aca8e7f34ac7985cca5b6ef12e7705f202
Signed-off-by: Billy O'Mahony<billy.o.mahony@intel.com>
Reviewed-by: Szilard Cserey <szilard.cserey@gmail.com>
Reviewed-by: Liang Gao <jean.gaoliang@huawei.com>