allow password to be set through WEAVE_PASSWORD env var
so that passwords can be supplied w/o them showing up in 'ps'
Merge branch 'master' into 276_alternative_password_supply
feed password to weave via env var, possibly via a file
The latter is the most secure route since it keeps the password
completely out of any command lines, and the permissions on the file
can be very restrictive, e.g. 0400.

It would be somewhat nicer if the password file contained just
    <password>
rather than
    WEAVE_PASSWORD=<password>
but then we'd have to transit the password via a temporary file with
0600 permissions.
Merge pull request #284 from rade/276_alternative_password_supply

LGTM.  Closes #276