MINOR: ssl/ckch: certificates generation from "load" "crt-store" directive

MINOR: ssl/ckch: certificates generation from "load" "crt-store" directive

Add "generate-dummy" on/off type keyword to "load" directive to
automatically generate dummy certificates as this is done for ACME from
ckch_conf_load_pem_or_generate() function which is called if a "crt"
keyword is also provide for this directive.

Also implement "keytype" to specify the key type used for these
certificates.  Only "RSA" or "ECDSA" is accepted. This patch also
implements "bits" keyword for the "load" directive to specify the
private key size used for RSA. For ECDSA, a new "curves" keyword is also
provided by this patch to specify the curves to be used for the EDCSA
private keys generation.

ckch_conf_load_pem_or_generate() is modified to use these parameters
provided by "keytype", "bits" and "curves" to generate the private key
with ssl_gen_EVP_PKEY() before generating the X509 certificate calling
ssl_gen_x509().