[DBAgentMySQL] Fix the problem in which the default value doesn't work.
[DBTables] Fix the wrong default value.

The previous patch fixes the bug of the default value and enables
it though, some default values of the string have been defiend in
a wrong format. The string default valud  should be quoted or NULL.
[DBTablesMonitoring] Add a new column 'valid' in a table 'hosts'.

The current implementation doesn't drop hosts records. However,
this behaivor is sometimes not hopeful. So we add a flag that is
indicating whether valid or not.
[HatoholArmPluginGate] Remove an unused variable.
[server] Define and set valid state for hosts.
[DBTablesMonitoring] Add a new method: updateHosts().

This patch also adds the followoing tests.
- test_updateHostsMarkInvalid().
- test_updateHostsAddNewHost().
[server] Use updateHosts.
[DBTablesMonitoring] Use a noun for a member instead of adjective.

valid ->
validity
[HostsQueryOption] Add method to take care of validity.
[testHostResourceQueryOptionSubClasses] Add a new test: test_hostsQueryOptionSetGetValidty().
[testHostResourceQueryOptionSubClasses] Add a new test: test_hostsQueryOptionGetConditionForHostValid().
[DBTablesMonitoring] Filter wanted hosts by the DB feature.
[Monitoring] Fix a typo

INAPPLICALBLE ->
INAPPLICABLE
Merge pull request #703 from project-hatohol/show-valid-hosts2

Show valid hosts2