Checks if the TLS secret contains a valid keypair structure

[rikatz]

This change checks if the concatenated file with keypair contains the expected keypair structure, with Certificate first and then the Private Key.

This is necessary to avoid errors as reported in issue #242

[coveralls]

Coverage increased (+0.005%) to 45.701% when pulling 02fbf00 on rikatz:issue-242 into fb8e2d7 on kubernetes:master.

[aledbf]

/lgtm

[k8s-reviewable]

This change is