url composition is broken is the wavefrontHost has a trailing slash.

For example specifying a config file like this does not work

Problematic
Wrting the following config to the config file at /Users/hbaba/.wavectl/config:
```
{
"apiToken": "9b9d16ef-3549-4e88-b4a2-f5e0736f2388",
"wavefrontHost": "https://box.wavefront.com/"
}

```

Working
```
{
"apiToken": "9b9d16ef-3549-4e88-b4a2-f5e0736f2388",
"wavefrontHost": "https://box.wavefront.com"
}

```

The difference is the trailing slash in the wavefronHost value.

I ran into this while using config files.
I am sure the same happens if the --wavefrontHost command line parameter is passed.