Fix volume filtering for quoted display name

Fix volume filtering for quoted display name

Cinder v2 API allows creating a volume with a quoted (single or double
quotes or even unbalanced number of quotes) display name. But when we
try to get info for such volume, we end up getting an error message
saying that no volume with such a name or ID exists. This error is due
to the inadvertent stripping of quotes from the filter in the api layer.

The api call eventually comes to check_volume_filters() in
cinder/volume/api.py. The invocation of ast.literal_eval() inside this
method strips the quotes for certain quoted strings leading to this
incorrect filtering. ast.literal_eval() is used to convert string
representations into python objects which are then used to frame the
SQL queries in the db layer. For example, the string "[1,2]" for a
filter (not the display name filter) gets converted to a list object
and results in an "IN" operation being emitted in the SQL query as
opposed to an exact match.

When display_name does not contain any quotes or contains an unbalanced
number of quotes, then ast.literal_eval() throws (just like the Python
interpreter would throw for an unquoted string literal or one with
unbalanced number of quotes). We handle this by ignoring the exception
and using the raw input value as the filter and moving on. For string
containing balanced number of quotes, such as, '"foo"',
ast.literal_eval() succeeds and returns the input with the surrounding
quotes stripped (just like how the python interpreter strips quotes
from a string literal to initialize a string var's value in memory).
To always use the raw user input string as the filter value, we can
either not pass string inputs to ast.literal_eval() or encode the
string using encode("string-escape") so that we get the original string
back after passing through ast.literal_eval(). We choose the former as
the latter buys us nothing.

Change-Id: I48e0aea801ccb011cb974eea3d685bb9f35c61b2
Closes-Bug: #1503485