Prevent password being referenced before assignment.

ALSO: prevent condition where r-w a DP mount is attempted w/o credentials resulting in exception

Hey Shea,
I put together a solution that worked for this...
https://github.com/lindegroup/autopkgr/issues/175

The particular issue here was that he only wanted a subset of his DPs to be used with jss-autopkg-addon,
so that there was only one DP and password in the JSS_REPOS array. 

I'm not sure if mounting all distribution points is a deliberate design decision (and it may serve a purpose of which I'm unaware), but this PR tries to pare down the list to ones that have either both username and password (read-write) or neither (read-only / guest)

You may have a different idea of how to solve it, but I wanted to put this out there for you.  
If you would like anything reworked please let me know.

--Eldon