Remove superfluous ssl context option which twisted already sets
Add certificate to extraCertChain

This commit is untested and probably won't work
because it may be an error to specify the same certificates in
`extraCertChain` as well as `certificate`...
Read cert chain from separate file
Add parsing of cert pem file

Here we use the pem module to parse our cert file into
certificate and chain... We call CertificateOptions constructor
with the private key, certificate and chain.
Fix handling of pem parse_file output
Fix parsing of key file.

Signed-off-by: Daira Hopwood <daira@leastauthority.com>
Refactoring and cosmetics.

Signed-off-by: Daira Hopwood <daira@leastauthority.com>