Added BIGIP license installation

[drenout]

Added license installation command similar to the revoking the license:

mgnt = ManagementRoot(bigip_mgnt, user, passwd)
mgnt.tm.sys.license.exec_cmd('install', registrationKey=regkey)

Issues:
Fixes #1483

Problem: It was not possible to install BIGIP license via f5-sdk

Analysis:
Added install command to the _meta_data['allowed_commands'] of the License object

Tests:

• Added unit test to make sure that install command is allowed and executed with the appropriate arguments
• Performed the functional tests with the hardware BIGIP:
  • With the correct license key, object returned by the mgnt.tm.sys.license.exec_cmd('install', registrationKey=regkey) command has 'commandResult': 'New license installed\n' attribute and BIGIP has installed regkey license
  • With the incorrect license key, object returned by the mgnt.tm.sys.license.exec_cmd('install', registrationKey=incorrect_regkey) command has 'commandResult': 'License server has returned an exception.\n Fault code: XXXX\n Fault text: Error XXXX, Production Key cannot be used in the Internal Development/PD environment - Key : XXXX\n' attribute and previous license key remains on the BIGIP

[jasonrahm]

@drenout can you change your two append lines to this line instead:

self._meta_data['allowed_commands'].extend(['revoke', 'install'])

[drenout]

@jasonrahm done. Thank you for the comment.