Add env variable SMIPATH as zProperty and update zenmib.py to use it

[kpg123]

It has been noted on the list that some people have had to manually add SMIPATH to their environment variables before running zenmib.py to add a MIB to Zenoss. By creating a zProperty (eg zSMIPATH), this environment can be more easily tracked and documented.

Suggested implementation in zenmib.py:

• if the environment variable SMIPATH exists, do nothing
• if the zProperty zSMIPATH is not null, export the variable SMIPATH before running smidump