[JENKINS-25576] A Ldap plugin test that runs with LDAP server is added.

There was only one Ldap plugin test which requires Docker to run. The
newly created LdapPluginNoDockerTest runs without Docker but with a
real LDAP server. Detailed configuration information can be set through
environment variables.

Because LdapPluginNoDockerTest has similar test stories with
ActiveDirectoryTest, ActiveDirectoryTest is refactored to reduce
duplicate code.