Move the metrics update to get_metrics

It is not necessary to update the metrics when get every metric in
one compute monitor plugin, just update once when get metrics for
the monitor plugin.

Change-Id: Ib665188964de8318f6d94343891666c674111c8c
Sync the latest DB code from oslo-incubator

This sync contains the following commits from olso-incubator:

7959826 db: move all options into database group
dda24eb Introduce mysql_sql_mode option, remove old warning
0b5af67 Introduce a method to set any MySQL session SQL mode
8dccc7b Handle ibm_db_sa DBDuplicateEntry integrity errors
0f24d82 Fix migration.db_version when no tables
ac84a40 Update log translation domains
c0d357b Add model_query() to db.sqlalchemy.utils module
84254fc Fix a small typo in api.py
b8a676c Remove CONF.database.connection default value
86707cd Remove None for dict.get()
0545121 Fix duplicating of SQL queries in logs
fcf517d Update oslo log messages with translation domains
fa05b7c Restore the ability to load the DB backend lazily
630d395 Don't use cfg.CONF in oslo.db
ce69e7f Don't store engine instances in oslo.db
35dc1d7 py3kcompat: remove
b4f72b2 Don't raise MySQL 2013 'Lost connection' errors
271adfb Format sql in db.sqlalchemy.session docstring
0334cb3 Handle exception messages with six.text_type
eff69ce Drop dependency on log from oslo db code
7a11a04 Automatic retry db.api query if db connection lost
11f2add Clean up docstring in db.sqlalchemy.session
1b5147f Only enable MySQL TRADITIONAL mode if we're running against MySQL
39e1c5c Move db tests base.py to common code
986dafd Fix parsing of UC errors in sqlite 3.7.16+/3.8.2+
bcf6d5e Small edits on help strings
ae01e9a Transition from migrate to alembic

Due to API changes in oslo.db code we have to change Nova code a bit
in order to reuse oslo-incubator changes (oslo.db no longer stores
SQLAlchemy Engine and sessionmaker instances globally and it's up to
applications to create them).

Change-Id: I4aaa7151f66e0292ff66c29330f93d78c6bf78a6
Merge "Move the metrics update to get_metrics"
Merge "Sync the latest DB code from oslo-incubator"