Add a warning to @memoized when caching is impossible

The @memoized decorator cannot cache function calls that take unhashable
arguments. So far it just ignored them, and silently called the function
directly without any cache. This commit adds a warning in such case, so
that we can see where @memoized is not really doing anything.

I used Python's warning mechanism, because we can easily configure the
particular warning to become an exception or to be ignored entirely.

Change-Id: I4b581f504972ffac589fdb78bfb9fb194d2b4525
Added help note in attach volume dialog

Added help note in Horizon's dashboard (Edit attachments dialog), in order to
inform that device name may not match the proposed one.
The intention is to provide more information when device names don't match due
to hypervisor limitations or naming convention.

Change-Id: I39828542047a2f2149815e2beba1099501aa755e
Partial-Bug: #1217874
Merge "Add a warning to @memoized when caching is impossible"
Merge "Added help note in attach volume dialog"