Allow customization of directory where cached credential data is stored

Currently, this directory is statically encoded as a subdirectory under `~/.aws`. However, it would be nice (for security reasons) to be able to store these credentials in another location. For example, when using MIT Kerberos on Linux, TGTs are stored in `/tmp`, and you can set the `KRB5CCNAME` env var to customize the filename of the active TGT cache (including changing the directory) (see `kinit(1)` man page).

Alternatively, as another example, it would be nice to store them in a directory in shared memory (e.g. `/dev/shm`) so that if the machine is powered off, the credentials would not persist.