E3 env selection

Allow users to specify the e3 environment they want to use for NFS iocs.

• Added ioc.epics_root, ioc.epics_version, and ioc.require_version options
• Added ioc_epics_root, ioc_epics_version and ioc_require_version so host-wide defaults can be set
• Updated role to use these variables/check that they are defined
• Provided a default ioc object
• Added some simple tests for e3 environment specification