limit value of nwa create tenant count is too low

[Shinji YANAGIDA]

Repeated nwa create tenant, it will not make a new tenant.
About 60 much only cannot make.

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/304921
Committed: https://git.openstack.org/cgit/openstack/networking-nec/commit/?id=e51f435ba5230c700e58253df64eddc81f8ef51e
Submitter: Jenkins
Branch: master

commit e51f435ba5230c700e58253df64eddc81f8ef51e
Author: Shinji YANAGIDA <email address hidden>
Date: Wed Apr 13 10:00:34 2016 +0900

    Fixes limitation of nwa create tenant

    There are two limitations.
    The one is on RPC and the other is on DB field size.

    The RPC limitation is that create_tenant_rpc_server did not return.
    create_tenant_rpc_server calls server.start(), but it did not return
    until the thread of callback is finished. Therefore, spawn an another
    thread for handling callback.

    The DB field size limitation is due to the size of information reported
    in PluginReportStateAPI. nwa agent reports a list of tenant IDs. When
    the number of active tenants becomes more than about 60, the size of
    information exceeds the column size. To reduce the size of information,
    the number of tenants is now reported.

    Change-Id: I332bff124a708eb41d272d95395bae8c7a028a69
    Closes-Bug: #1569628