Don't force the Kerberos cache to be stored as a file when using the ccache kwarg on krb_login
A multi-threaded application may choose to store the Kerberos cache in the
thread keyring [1] to avoid Kerberos cache corruption. Since `krb_login` prepends
the passed in `ccache` with `FILE:`, the application must resort to setting the
`KRB_CCACHE` environment variable. This is annoying and unnecessary because
ccache defaults to `FILE` anyways if no Kerberos cache type is specified in the
value for `ccache` [2].
1 - http://man7.org/linux/man-pages/man7/thread-keyring.7.html
2 - https://web.mit.edu/kerberos/krb5-1.12/doc/basic/ccache_def.html#ccache-types