Add a mechanism to swap nailgun versions on demand

Satellite._swap_nailgun("x.y.z") can be used to change out the operating
version of nailgun.

(cherry picked from commit f10676e)

robottelo/hosts.py

@@ -1740,14 +1740,26 @@ def __init__(self, hostname=None, **kwargs):
         self._api = type('api', (), {'_configured': False})
         self._cli = type('cli', (), {'_configured': False})
 
+    def _swap_nailgun(self, new_version):
+        """Install a different version of nailgun from GitHub and invalidate the module cache."""
+        import sys
+
+        from pip._internal import main as pip_main
+
+        pip_main(['uninstall', '-y', 'nailgun'])
+        pip_main(['install', f'https://github.com/SatelliteQE/nailgun/archive/{new_version}.zip'])
+        self._api = type('api', (), {'_configured': False})
+        to_clear = [k for k in sys.modules.keys() if 'nailgun' in k]
+        [sys.modules.pop(k) for k in to_clear]
+
     @property
     def api(self):
         """Import all nailgun entities and wrap them under self.api"""
         if not self._api:
             self._api = type('api', (), {'_configured': False})
         if self._api._configured:
             return self._api
-
+        from nailgun import entities as _entities  # use a private import
         from nailgun.config import ServerConfig
         from nailgun.entity_mixins import Entity
 
@@ -1767,7 +1779,7 @@ class DecClass(cls):
             verify=settings.server.verify_ca,
         )
         # add each nailgun entity to self.api, injecting our server config
-        for name, obj in entities.__dict__.items():
+        for name, obj in _entities.__dict__.items():
             try:
                 if Entity in obj.mro():
                     #  create a copy of the class and inject our server config into the __init__