The Elasticache API accepts a mixed-case subnet name on create, but
normalizes it to lowercase before storing it. When retrieving a subnet,
the name is treated as case-sensitive, so the lowercase version must be
used.
Given that case within subnet names is not significant, the new StateFunc
on the name attribute causes the state to reflect the lowercase version
that the API uses, and changes in case alone will not show as a diff.
Given that we must look up subnet names in lower case, we set the
instance id to be a lowercase version of the user's provided name. This
then allows a later Refresh call to succeed even if the user provided
a mixed-case name.
Previously users could work around this by just avoiding putting uppercase
letters in the name, but that is often inconvenient if e.g. the name is
being constructed from variables defined elsewhere that may already have
uppercase letters present.
