Keystone/LDAP integration

Installs python-ldap and ldappool modules into Keystone virtual environment.

Configuration settings related to LDAP are supposed to be set via
passthrough feature (i.e. injected into /etc/keystone/keystone.conf file
by setting "keystone" field of $OVERCLOUD_EXTRA_CONFIG env variable).

Change-Id: Ifcd403cff3a75a9926d2a4ee7d041ef9f3171ffb
Merge "Keystone/LDAP integration"