Added env JOBSUB_MANAGED_TOKEN to control --managed-token flag default

[shreyb]

This PR adds an environment variable, JOBSUB_MANAGED_TOKEN, corresponding to the new --managed-token flag, that allows users to set that flag in the environment

# Default behavior
$  bin/jobsub_submit -G fermilab -n --debug file:///usr/bin/true
...
varg: {...  'managed_token': False, 'memory': '2GB', ...}
...

# With flag
$  bin/jobsub_submit -G fermilab -n --debug --managed-token file:///usr/bin/true
...
varg: {... 'managed_token': True, 'memory': '2GB', ...}
...

# With env
$ JOBSUB_MANAGED_TOKEN=1 bin/jobsub_submit -G fermilab -n --debug file:///usr/bin/true
...
varg: {...  managed_token': True, 'memory': '2GB', ...}
...

There are also a couple of tests that check this.

[marcmengel]

I like it!