Abstract out attribute_ignore assigning in LDAP driver

This is used to avoid code duplication in keystone/identity/
backends/ldap.py. All changes are fully covered by test_*option_
name*_attribute_ignore() in test_backend_ldap.py.

bp refactor-ldap-driver

Change-Id: Ia0fc0b3d4a92416dcd65ddb49b3b0bf9a0777363
Merge "Abstract out attribute_ignore assigning in LDAP driver"