Improve I/O performance for periodic tasks

Unfortunately, periodic tasks to update local resource and
driver status come with performance penalties, especially on LVM
image backend type.

This is because low level implementation relies 'vgs' command to
query LVM info, which itself has a I/O performance price and
extravagant use of local_gb_xxx query make things even worse.

This commit centralize such query in one single call
(volume_group_info) and reduce invocation times in periodic tasks
accordingly.

Partially resolves LP #1130697

Change-Id: I165f96dd3e4fdad770b8a9643f5a038d353312ff
Merge "Improve I/O performance for periodic tasks"