Adding support for ldap connection pooling.

Using ldappool library to establish connection pooling.
Connection pooling is disabled by default.
Pooling specific configuration parameters are added in ldap section.
Added pool test using existing FakeLdap as connector class.
Added pool specific ldap live test. These tests are executed similar to
existing ldap live test.
Addressed async search_s and result3 API issues mentioned in review.
Added separate connection pool for end user auth bind done by keystone
identity ldap driver logic to avoid saturation of pool by these kind of
binds and limiting pool effectiveness for other ldap operations.
Rebased with lastest master and addressed doc comments.

Change-Id: If516a0d308a7f3be88df5583a30739a935076173
Closes-Bug: #1320997
bp: ldap-connection-pooling
DocImpact