BUG/MINOR: jwt: don't try to load files with HMAC algorithm

BUG/MINOR: jwt: don't try to load files with HMAC algorithm

When trying to use a HMAC algorithm (HS256, HS384, HS512) the
sample_conv_jwt_verify_check() function of the converter tries to load a
file even if it is only supposed to contain a secret instead of a path.

When using lua, the check function is called at runtime so it even tries
to load file at each call... This fixes the issue for HMAC algorithm
but this is still a problem with the other algorithms, since we don't
have a way of pre-loading files before the call.

Another solution must be found to prevent disk IO with lua using other
algorithms.

Must be backported as far as 2.6.