Sync the DB2 communication error code change from olso

Closes-Bug: #1244092

oslo patch set: 3acd57c2825a8bf6660c3124f12794414622eb26

Change-Id: I105ff3691804c6c77548d3250cc241293397187f
PasteConfigNotFound also raised when keystone.conf not found

Although keystone is looking for paste configuration on startup, this
exception can also be raised when keystone attempts to look in the
regular keystone.conf file for paste configuration, and can't find that
file either. The solution here is just to make the exception slightly
more generic (ConfigFileNotFound), along with it's rendered error
message and improve surrounding comments to better explain the scenario.

Change-Id: I88f1d987113884097b1cdf8c120613613abfb8c9
Closes-Bug: 1254036
Merge "PasteConfigNotFound also raised when keystone.conf not found"
Merge "Sync the DB2 communication error code change from olso"