Add apt.upgrade() convenience function

this one just does the 'upgrade', but *not* the 'update'

rattail_fabric/apt.py

@@ -61,6 +61,16 @@ def update():
     sudo('apt-get update')
 
 
+def upgrade(frontend='noninteractive'):
+    """
+    Perform an ``apt-get upgrade`` operation.
+    """
+    options = ''
+    if frontend == 'noninteractive':
+        options = '--option Dpkg::Options::="--force-confdef" --option Dpkg::Options::="--force-confold"'
+    sudo('DEBIAN_FRONTEND={} apt-get --assume-yes {} upgrade'.format(frontend, options))
+
+
 def add_repository(repo):
     """
     Add a new APT repository