Have broctl return a useful exit code

Applied patch from Vlad Grigorescu.

Addresses #920.
Previously-applied patch needs a small adjustment
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.
Previously-applied patch needs another small fix

Patch applied in commit 4274c596d7429c299467130d8816ea97aebe25bc needed
another small fix.
Minor bug fix of the broctl process command

Added a missing "return" statement to prevent trying to run Bro when
there is a syntax error with the broctl "process" command.

Also removed some redundant lines that were setting a non-zero exit code
twice.
Fix broctl exit status for capstats command

Fixed the broctl capstats command to report a non-zero exit status
when an error is encountered.
Previously-applied patch needs another small fix

Patch applied in commit 4274c596d7429c299467130d8816ea97aebe25bc needed
another small fix (a variable that is always a boolean was being checked
for equality to a string).
Merge remote-tracking branch 'origin/fastpath'

* origin/fastpath:
  fix broken link (thanks kraigu)
Add example usage of env_vars in documentation
Add error reporting to more broctl commands

Added error reporting (via a non-zero return code) to the cleanup,
df, install, and print commands.  Also removed some redundant lines
of code in restart command that were setting error code more than once.
Updating submodule(s).

 [nomail]
Merge remote-tracking branch 'origin/master' into topic/dnthayer/bug920
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.
Merge remote-tracking branch 'origin/master' into topic/dnthayer/bug920
Updating submodule(s).

 [nomail]
Merge remote-tracking branch 'origin/topic/dnthayer/bug920'

BIT-920 #merged

* origin/topic/dnthayer/bug920:
  Add error reporting to more broctl commands
  Previously-applied patch needs another small fix
  Fix broctl exit status for capstats command
  Minor bug fix of the broctl process command
  Previously-applied patch needs another small fix
  Previously-applied patch needs a small adjustment
  Have broctl return a useful exit code