Honor EUCALYPTUS environment variable in eucacore plugin.

[risaacson]

In the case of a build from source the eucalyptus proper application could be installed in an alternate location. In that case the standard is that there will be a EUCALYPTUS environment variable set that will point to the eucalyptus root.

The changes for this should be limited strictly to the eucacore plugin and will not affect euca2ools or our pgsql install at this point.

-r

[risaacson]

Prior art to target: https://github.com/eucalyptus/eucalyptus/blob/master/tools/eucalyptus-nc.in lines 115 - 122