Fuel agent tiny fixups

1) Fix issue with /etc/mtab for fuel-agent

Sometimes /etc/mtab inside of chrooted image during provisioning stage
appears as symlink to /proc/self/mounts. It can't be rewritten in such
case. So its removing really helps.

2) Fix grub_utils

`chroot` parameter was missed few times. Should work now

3) Fix improper stdout handling in manager.py when `blkid` was called

Output should be stripped as it contains new-line symbol (\n) on end
Closes-Bug: #1383371

Change-Id: I756b85b104fc06a065bc2f297b30f0b738aea3f4
Merge "Fuel agent tiny fixups"