Config refactor

[gravestench]

depends on #33!

• config global state removed from the config module
• config global state now resides in main, TODO to remove it
• config now creates a default if none are found
• added command line option for specifying the config file
• added command line option to specify the log level
• config directory now looks in user dir, in a platform agnostic way.
  ex: on linux, will typically look in ~/.config/my5g/RANTester for file
  config.yml
• updated go.mod to use go v1.16
• embedded default config file data
• errors are now bubbled up appropriately

[gravestench]

running now looks like this:

go run ./cmd/my5g-RANTester/

NAME:
   my5g-RANTester - A new cli application

USAGE:
   my5g-RANTester [global options] command [command options] [arguments...]

COMMANDS:
   ue         Testing an UE attached with configuration
   gnb        Testing a GNB attached with configuration
   load-test  Load endurance stress tests.
              Example for testing multiple UEs: load-test -n 5
              
   help, h    Shows a list of commands or help for one command

GLOBAL OPTIONS:
   --loglevel value, -l value  set the log level of the app, (6 is most verbose) (default: 4)
   --config value, -c value    set the config file to use
   --help, -h                  show help (default: false)
exit status 1

[gravestench]

here's what running with a command looks like:

go run ./cmd/my5g-RANTester/ -l=6 ue

DEBU[0000] setting log level to trace                   
INFO[0000] using config file at: /home/dk/.config/my5g/RANTester/config.yml 
DEBU[0000] config loaded successfully                   
INFO[0000] ---------------------------------------      
INFO[0000] [TESTER] Starting test function: Testing an ue attached with configuration 
INFO[0000] [TESTER][UE] Number of UEs: 1                
INFO[0000] [TESTER][GNB] Control interface IP/Port: 192.168.10.108/9487 
INFO[0000] [TESTER][GNB] Data interface IP/Port: 192.168.10.108/2152 
INFO[0000] [TESTER][GNB] AMF IP/Port: 192.168.122.83/38412 
INFO[0000] ---------------------------------------      
FATA[0000] Error incannot assign requested address

[gravestench]

here's what default verbosity looks like, with an invalid config path specified:

go run ./cmd/my5g-RANTester/ --config=/foo/bar.yaml ue

WARN[0000] could not find config file '/foo/bar.yaml'   
INFO[0000] config file exists                           
INFO[0000] using config file at: /home/dk/.config/my5g/RANTester/config.yml 
INFO[0000] ---------------------------------------      
INFO[0000] [TESTER] Starting test function: Testing an ue attached with configuration 
INFO[0000] [TESTER][UE] Number of UEs: 1                
INFO[0000] [TESTER][GNB] Control interface IP/Port: 192.168.10.108/9487 
INFO[0000] [TESTER][GNB] Data interface IP/Port: 192.168.10.108/2152 
INFO[0000] [TESTER][GNB] AMF IP/Port: 192.168.122.83/38412 
INFO[0000] ---------------------------------------      
FATA[0000] Error incannot assign requested address      
exit status 1