fix raise of UndefinedVariableError

[epcim]

When used in Salt, as of now it only printed:

root@cfg01:/usr/lib/python2.7/dist-packages/reclass# salt ctl01\* pillar.data 
ctl01.virtual-mcp10-ovs.local:
    ----------
    _errors:
        - Failed to load ext_pillar reclass: ext_pillar.reclass: 

With this patch:

root@cfg01:/usr/lib/python2.7/dist-packages/reclass# salt ctl01\* pillar.data 
ctl01.virtual-mcp10-ovs.local:
    ----------
    _errors:
        - Failed to load ext_pillar reclass: ext_pillar.reclass: Cannot resolve ${_param:swift_service_host} in the context of keystone:client:service:swift-s3:endpoints:0:internal_address