fix: key/cert switchup for rtr-tls target

[ember-ana]

when defining a target with type = "rtr-tls", the config keys certificate and key refer to the respective other

actual

assuming test-data/localhost.crt and test-data/localhost.key

[targets.rtr-tls]
type = "rtr-tls"
certificate = "localhost.crt"
key = "localhost.key"

will fail with: TLS key file 'localhost.crt' does not contain any usable keys.

meanwhile

[targets.rtr-tls]
type = "rtr-tls"
key = "localhost.crt"
certificate = "localhost.key"

will start successfully

expected

[targets.rtr-tls]
type = "rtr-tls"
certificate = "localhost.crt"
key = "localhost.key"

should start successfully

[partim]

Thank you for the PR! Looks perfect! (And also: oops.)