Fix the argument of UnknownArgument exception

This change sets the real UnknownArgument instead of the last
valid one in the exception raised by the API.

Fixes bug #1201839

Change-Id: I22e1dadbe9a90e03e8ccfbcc6e0e8dc3f17c5a3e
doc: add a bunch of functional examples for the API

Change-Id: I364208c87e450e7cb0443774e80916845dbfb32a
Add index for db.meter by descending timestamp

Fixes bug #1193906

Added an index for db.meter to sort by descending time stamp for class
Connection in ceilometer/storage/impl_mongodb.py

ensure_index() is used rather than create_index() to be more efficient.

This index is needed because get_samples() in
ceilometer/storage/impl_mongodb.py sorts by descending timestamp.  Without an
index, MongoDB will fail to sort if the database is too large.

Change-Id: I23070f153ce03a8d6d8c9f17f05e2d5dae38647e
Merge "doc: add a bunch of functional examples for the API"
Merge "Fix the argument of UnknownArgument exception"