XD-1982 Add https Support to http Source

Tested as follows:

- Obtained an SSL certificate from startssl.com (free) for my domain
- Installed into a keystore along with startssl's trusted and root certs...

````
gpr@testserver:~/Documents/startssl$ cat testserver.pem sub.class1.server.ca.pem ca.pem > chain.pem
gpr@testserver:~/Documents/startssl$ openssl pkcs12 -export -in chain.pem -inkey ssl.key -out testserver.pkcs12
````

- Installed the intermediate and new cert into java's cacerts...

````
gpr@testserver:/usr/lib/jvm/jdk1.7.0_51/jre/lib/security$ sudo keytool -import -trustcacerts -keystore cacerts -alias testserver -file ~/Documents/startssl/testserver.cer
gpr@testserver:/usr/lib/jvm/jdk1.7.0_51/jre/lib/security$ sudo keytool -import -trustcacerts -keystore cacerts -alias startsslsubclass1 -file ~/Documents/startssl/sub.class1.server.ca.pem
gpr@testserver:/usr/lib/jvm/jdk1.7.0_51/jre/lib/security$ sudo keytool -import -trustcacerts -keystore cacerts -alias startsslroot -file ~/Documents/startssl/ca.pem
````

(Default password for java cacerts `changeit`)

The startssl.com root cert was already in the cacerts.

Add fully qualified hostname for the server for which the cert was created to `/etc/hosts`

- Start singlenode
- Start shell

````
xd:>stream create foo --definition "http --https=true --secrets=file:/home/gpr/tmp/httpssl | log" --deploy
Created and deployed new stream 'foo'
xd:>http post --target https://testserver.mydomain.net:9000 --data "hello"
> POST (text/plain;Charset=UTF-8) https://testserver.mydomain.net:9000 hello
"~/Development/spring-xd/.git/COMMIT_EDITMSG" 51L, 2227C
> 200 OK
````

````
17:57:11,138  INFO pool-11-thread-7 sink.foo - hello
````


Ah, I get it. So we do NOT want to imply SSL only because the properties file exists. Rather IF one asks for SSL, they will need to be sure to have the ssl location also (and there is a default location). Makes sense. Let's keep it as is.