Start to fill out LDAPIdentityProvider's fields and TestSupervisorLogin

- Add some fields to LDAPIdentityProvider that we will need to be able
  to search for users during login
- Enhance TestSupervisorLogin to test logging in using an upstream LDAP
  identity provider. Part of this new test is skipped for now because
  we haven't written the corresponding production code to make it
  pass yet.
- Some refactoring and enhancement to env.go and the corresponding env
  vars to support the new upstream LDAP provider integration tests.
- Use docker.io/bitnami/openldap for our test LDAP server instead of our
  own fork now that they have fixed the bug that we reported.

Signed-off-by: Andrew Keesler <akeesler@vmware.com>

test/deploy/tools/ldap.yaml

@@ -140,9 +140,7 @@ spec:
     spec:
       containers:
         - name: ldap
-          #! An issue was reported and will be fixed in bitnami/openldap soon.
-          image: ghcr.io/pinniped-ci-bot/bitnami-openldap-forked:2.4.58-debian-10-r15 #! our own fork of docker.io/bitnami/openldap
-          #! image: docker.io/bitnami/openldap
+          image: docker.io/bitnami/openldap
           imagePullPolicy: Always
           ports:
             - name: ldap
@@ -182,9 +180,6 @@ spec:
               value: "/var/certs/ldap-key.pem"
             - name: LDAP_TLS_CA_FILE
               value: "/var/certs/ca.pem"
-              #! This env var was added in our fork to reduce slapd memory consumption from ~700 MB to ~12 MB.
-            - name: LDAP_ULIMIT_MAX_FILES
-              value: "1024"
               #! Note that the custom LDIF file is only read at pod start-up time.
             - name: LDAP_CUSTOM_LDIF_DIR
               value: "/var/ldifs"