Fixes file mode bits of compute/manager.py

Modified file permission from 755 to 644.
Removed a redundant blank line to avoid merged errors in gerrit
which happens if no content has changed.

Partially fixes bug #1224226

Change-Id: Ia300ac05fbeb251bb801cec44011df94ce8f16b0
Merge "Fixes file mode bits of compute/manager.py"
Move call to _default_block_device_names() inside try block

During instance builds, a call to _default_block_device_names() is
made which contains a db call to update the instance. In case the
instance is gone it will throw an unhandled InstanceNotFound exception.

To properly handle this, move the call to _default_block_device_names()
inside the existing try block next to it that handles this exception.

Change-Id: I3503029f02669e6ae31441653096fa69cba76d45
Closes-Bug: #1224677