Add ability to set environment variables in node.cfg

Added the ability to specify environment variables in node.cfg
to be set when Bro is started (these can override values set by
the load-balancing plugins).  This uses the existing "env_vars"
node attribute, but now allows a user to specify it in the node.cfg file.

One limitation is that the value of any such environment variable must
be a constant.  If a value can be determined only at run-time, then a user
would need to write their own plugin to set such a variable (similar to
the load-balancing plugins).

Addresses #1010.
Add example usage of env_vars in documentation
Change implementation of env_vars attribute

Now env_vars is stored internally as a dictionary, and the env_vars
configure option in node.cfg is now changed to a comma-separated
list.  As a result, there is now more error-checking of the
user-supplied string in node.cfg, and there is no longer
any possibility for duplicate environment variables to appear
in debug.log or in the "broctl nodes" output (which should reduce
confusion).
Add env_vars option to broctl.cfg

Added a new option to broctl.cfg that works the same way as the "env_vars"
option in node.cfg, except that an environment variable in node.cfg
overrides the same environment variable (if specified) in broctl.cfg.
Merge remote-tracking branch 'origin/master' into topic/dnthayer/ticket1010
Merge remote-tracking branch 'origin/topic/dnthayer/ticket1010'

* origin/topic/dnthayer/ticket1010:
  Add env_vars option to broctl.cfg
  Change implementation of env_vars attribute
  Add example usage of env_vars in documentation
  Add ability to set environment variables in node.cfg

BIT-1010 #merged
Test fixes.

White-space and sorting of environment variables were inconsistent
with base lines.