"nova boot" should not log an error if subsidiary commands fail

Fix:
Intially the cli was raising "CommadError" in case the requested
flavor or image were not present.
This error category was not approrpiate as it signifies an error
in the command syntax. When the requested resource (flavour/image)
does not exist, a ResourceNotFound error should be raised. So,
added a new error category "ResourceNotFound" to cater for this
scenario and updated the code to raise this new error.
"nova show <instance_name>" command has also been updated to raise
"ResourceNotFound" error when the requested vm for which details
have to be displayed does not exist.

Closes-Bug: #1258488
Change-Id: If64a087944586ef5792efe3baa62e455b9bbaa07
Look for all accessible flavors by default, not just public ones

The Nova server will restrict unprivileged user accounts to just
public images, while allowing administrator accounts access to
all. The Nova client shouldn't force the flavor name lookup to
be restricted to just public images, since that breaks the ability
to the flavor name when booting an instance

Fixes bug #1327212

Change-Id: I949aec52660242249b8cba51d77bfdc1acaf31d2
Merge ""nova boot" should not log an error if subsidiary commands fail"
Merge "Look for all accessible flavors by default, not just public ones"