Update Measurement Docs for LBaaS

Partially Implements: blueprint ceilometer-meter-lbaas

Change-Id: I3f2c0590ef665405c15b7c01523003af7042538b
Skipping central agent pollster when keystone not available

When keystone is not available, instead of skipping all central agent
pollsters, we should let the pollsters decide whether to skip or not,
because not all central agent pollsters requires keystone.

Closes-Bug: #1316532

Change-Id: Ib34f9838bd128e1b357937f0c177d8a09f19b13f
[HBase] Implement events on HBase

In this change events are written into HBase and read from it.

For this purpose were implemented several methods:
1. record_events --- store events into HBase
2. get_events, get_event_types, get_trait_types and get_traits --- get events
from db.

Add new filters in mock HBase part, that were used in new methods.

Change-Id: I787eeaf998c36a6829f17f0c6ea27b786979edd1
Implements: blueprint hbase-events-feature
Merge "Update Measurement Docs for LBaaS"
Merge "[HBase] Implement events on HBase"
Merge "Skipping central agent pollster when keystone not available"