Fix invalid read_deleted value in _validate_unique_server_name()

read_deleted passed to model_query() may have the value 'yes', 'no',
or 'only', but was being passed the value False. The implementation of
model_query caused this to be ignored, and the value of
context.read_deleted used instead.

This change cleans up kwarg parsing in model_query to ensure that all
values explicitly passed in retain their given value, unless they were
explicitly set to None, in which case they are defaulted anyway.  This
last is the expectation of current callers of model_query().

Changes Exception thrown for invalid values to ValueError.

Change-Id: I2d1bbdbd4a391ad3487da905df2e812166a837de
Merge "Fix invalid read_deleted value in _validate_unique_server_name()"