Add license installation

Add license installation command similar to the revoking the license: 

Code snippet for the revoking the license:
```python
mgnt = ManagementRoot(bigip_mgnt, user, pass)
mgnt.tm.sys.license.exec_cmd('revoke')`
```
Code snippet for the license installation:
```python
mgnt = ManagementRoot(bigip_mgnt, user, pass)
mgnt.tm.sys.license.exec_cmd('install', registrationKey=regkey)
```