Command-line flags should take precedence over OS Env variables

Currently, it appears that re-defining a flag on the command-line that was already defined via a OS Env var (MESOS_*) causes the Master to fail with a not very helpful message.

For example, if one has MESOS_QUORUM defined, this happens:

$ ./mesos-master --zk=zk://192.168.1.4/mesos --quorum=1 --hostname=192.168.1.4 --ip=192.168.1.4
Duplicate flag 'quorum' on command line

which is not very helpful.

Ideally, we would parse the flags with a "well-known" priority (command-line first, environment last) - but at the very least, the error message should be more helpful in explaining what the issue is.