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
````


Why do we need the boolean, as opposed to checking if sslPropertiesLocation is null?