Provide ability to modify META_COLL_ATTR_UNITS

[rwmoore]

Needed for modifying tag metadata

[rwmoore]

Using the iPlant collaborative micro-service

          msiSetAVU ("-d", *Dest1, *Attn, *Attv, *Attu);

[d-w-moore]

Now modifying the 3.x microservice msiSetAVU to wrap rsModAVUMetadata with a microservice

msiModAVUMetadata( *objectType, *objName, *operation, *attName,*attValue,*attUnits )

where operation can be one of add, set, rm, or rmw. Planning it as part of the 4.2.7 release.

[trel]

Done in #4521