feat(dev): improve Rocky Linux and RHEL compatibility for local development

- Relax libvirt-python and lxml version pins to allow precompiled distro packages
- Add PEP 508 environment marker to crypt-r (python_version >= 3.13) to avoid unnecessary C compilation on Python 3.12
- Add stdlib crypt fallback when crypt_r is not available in instances/views.py
- Make LDAP imports and configuration optional in settings.py.template
- Add --system-site-packages and missing packages in webvirtcloud.sh for RHEL/CentOS
- Update README with working Rocky Linux / RHEL / Fedora installation and local dev setup

conf/requirements.txt

@@ -11,8 +11,8 @@ drf-spectacular[sidecar]==0.28.0
 eventlet==0.40.1
 gunicorn==23.0.0
 libsass==0.23.0
-libvirt-python==11.4.0
-lxml==6.0.0
+libvirt-python>=11.0.0
+lxml>=5.2.0
 ldap3==2.9.1
 markdown==3.8.2
 paramiko==3.4.0
@@ -24,4 +24,4 @@ tzdata
 websockify==0.13.0
 whitenoise==6.9.0
 zipp==3.23.0
-crypt-r==3.13.1
+crypt-r>=3.13.0; python_version >= "3.13"