Fixes insecure update of /etc/fstab file

Reasons:
- While updating /etc/fstab in write_to_fstab in
  trove/guestagent/volume.py, the temporary file-name was easy to
  guess and had a permanent name making it prone to unwanted access.
- The temporary file-permission was set to 666, making it insecure.

Changes:
- Uses NamedTemporaryFile for temporary file creation.
- Used file reading instead of copying the file.
- These changes eliminate the chances of guessing the file-name,
  as well as updating the same.

Change-Id: I07c3980e40fe218d35605a851a17535cd3972c11
Closes-Bug: #1267991
Merge "Fixes insecure update of /etc/fstab file"