support for license parameter in easyconfig

The license parameter is supported but not mandatory. More license classes need to be added first. The value in the easyconfig is a constant, not a string ie
```
license=GPLv2
```
not 
```
license='GPLv2'
```

Available licenses can be checked with 
```
eb --avail-easyconfig-licenses
```

This is a techpreview, ie subject for discussion and improvement.