MINOR: ssl: add the section parser for 'crt-store'

MINOR: ssl: add the section parser for 'crt-store'

'crt-store' is a new section useful to define the struct ckch_store.

The "load" keyword in the "crt-store" section allows to define which
files you want to load for a specific certificate definition.

Ex:
    crt-store
        load crt "site1.crt" key "site1.key"
        load crt "site2.crt" key "site2.key"

    frontend in
        bind *:443 ssl crt "site1.crt" crt "site2.crt"

This is part of the certificate loading which was discussed in #785.