Made 'pxf (cluster) prepare' command work in a multi-cluster architecture

[denalex]

This PR makes the PXF CLI and the pxf bash script support a multi-cluster architecture. In this first phase the functionality is limited to having only 1 cluster with the name "default". Therefore, a "cluster name" parameter is not added at this time.

Changes include:

• pxf bash script prepare command now creates $PXF_BASE/clusters/default directory and its subdirectories and copies templates from $PXF_HOME/templates/cluster and $PXF_HOME/templates/conf into respective subdirectories.
• $PXF_BASE value set by user is now allowed to be the same as $PXF_HOME value
• PXF CLI go tool now requires both PXF_BASE and PXF_HOME` environment variables to be set, which they are if it is invoked from the pxf bash script
• internal refactoring of the PXF CLI go tool and unit tests
• PXF CLI go tool now supports running commands across both Greenplum and PXF external clusters with the reporting of status indicating the count of PXF external hosts the command will run on. The outcome reporting also include the count of PXF external hosts as well as any errors that might have occurred there.