Update installation.rst
Update installation.rst

Updated the paths to cloud-install-sys-tmplt
Updated links to system templates using values from the upgrade from 4.2.1 to 4.3 release notes

This closes #6

pdion891/cloudstack-docs-install,unodba/cloudstack-docs-install,apache/cloudstack-docs-install,unodba/cloudstack-docs-install,pdion891/cloudstack-docs-install,pdion891/cloudstack-docs-install,apache/cloudstack-docs-install,apache/cloudstack-docs-install,unodba/cloudstack-docs-install