twiddle ignores environment properties when creating its InitialContext

The original feature request:
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It is frequently useful to be able to set JDNI properties when running twiddle - for example,
you might want to set jnp.disableDiscovery when checking the status of a particular server
that might not be running.

Right now, this is difficult to do. You can export a JBOSS_CLASSPATH
which contains a jndi.properties file before running twiddle.sh, but then you have to
include all the standard jars in it, which is quite annoying.

A pssible fixes would be one of:

A) A way to prepend elements to JBOSS_CLASSPATH without disabling the default
construction (also useful for getting a log4j.properties that doesn't drop twiddle.log
in the cwd, perhaps)

B) A command line option for specifying a particular properties file to load and pass
to 'new InitialContext()' (slightly more convenient than A)

C) A command line option to define particular properties:

twiddle.sh -jnpProperty=jnp.disableDiscovery=true

(Or some better syntax)

The twiddle code actually does:

Properties props = new Properties(System.getProperties());
props.put(Context.PROVIDER_URL, serverURL);
ctx = new InitialContext(props);

In the case where a server URL is set, so there may have been intention that twiddle.sh -Djnp.disableDiscovery=true
would work, but AFAIK property inheritance doesn't work in that place - only the properties directly in props take
effect, and system properties won't be looked at in the other code path either.
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The real problem is that twiddle ignores environment properties when creating its InitialContext.

By fixing this you can just specify in the command line:
-Djnp.disableDiscovery