MAR-181: Drop support for Debian 9 Stretch from ldap_server role:

- Switch to using IPs from VirtualBox default allowed host-only
network subnets.
- Drop the workaround for unsupported TLS ciphers on Debian Stretch.

MAR-165: Deploy Diffie-Helman parameters for LDAP server in the ldap_server role:

- Not relevant for Debian Strech because of a bug in the OpenLDAP
version it ships with.
- This should allow use of DHE ciphers with LDAP server.
- Generated DH parameters only help pick one of the parameters from
RFC-7919 (based on the size of generated ones).
- Make the cipher test lists distro-specific due to differences
between supported algorithms in respective GnuTLS versions.

MAR-158: Refactor ldap_server TLS-related tests to use nmap:

- Updated requirements to include defusedxml for safe parsing of XML
reports from nmap.
- Install nmap as part of preparation step.
- Refactored tests for TLS to use nmap ssl-enum-ciphers script for
listing available TLS versions and ciphers.

MAR-158: Update default TLS ciphers configuration in the ldap_server role:

- Updated the default value for parameter ldap_tls_ciphers.
- Updated tests, making them explicitly test for enabled and disabled
ciphers
- Updated role reference documentation.

MAR-162: Deduplicate TLS private key/certificate tests for LDAP server role:

- Rename the key/certificate files to match the Ansible inventory
name.
- Move the tests into test_default.py.
- Change the key/certificate extensions to be more descriptie.

MAR-149: Switch to using File.content_string instead of File.content in tests for all roles:

- The .content variant returns contents as bytes, while the
content_string returns a string (which is what is wanted in
practically all cases).

MAR-132: Parametrise tests in ldap_server role:

- Dynamically obtain inventory hostnames (don't hard-code them).
- Parametrise connectivity test, switching to using inventory
hostnames along the way (instead of hard-coded IPs in tests.
- Populate /etc/hosts with inventory hostnames in client machine (used
for connectivity tests).

MAR-129: Updated ldap_server role for new Molecule and Ansible:

- Fixed syntax warnings.
- Moved test playbook variables into group_vars.
- Optimised the invocation of test playbook.
- Remove the 'ss' utility from test machines in order to properly test
the unix sockets.
- Fixed how the hosts are being referenced in tests.

MAR-128: Upgraded tests for ldap_server role:

- Switch to new Molecule configuration.
- Updated set-up playbook to use become: yes.
- Moved some preparatory steps outside of the main playbook (eases
idempotence tests).
- Updated tests to reference the yml inventory file.
- Updated tests to use new fixture (host instead of individual ones).
- Fixed some linting issues.
- Use localhost as backup client server name. This should make it
simpler in future to cover more platforms.
- Set-up the /etc/hosts for tests with correct entries (primarily
aliases towards localhost to make it less important what the
hostname is).
- Renamed private key/certificate files for parameters-mandatory
server since server got renamed as well.
- Updated IP addresses used for testing to make it easier to test new
platforms in the future.
- Use inventory_hostname-based parameterisation of tests where
appropriate. Currently hard-coded value for this because
host.ansible.get_variables() produces errors.