No task add config subcommand

I had an idea as to something that would make creating a credentials config file easier for new users.
I have added a config subcommand that allows a user to read and write to the login.yml file

The two commands are:
jenkinscli config add - Adds the given credentials to the login.yml
```
$ jenkinscli config add -u name -p password -s 1.2.3.4
```

jenkinscli config list - Lists the current login.yml contents
```
$ jenkinscli config list
username: name
password: password
server_ip: 1.2.3.4
```

During running the unit tests some failed due to a change in rspec. The be_true matcher now needs to be be_truthy. I have updated the failing tests in my second commit.