install_certificate: change the behaviour for $privatekey=true/false.

before it used $privatekey to make a decision about the location
of the private key. it was either files/ssl/ or private/ssl.

the files/ssl part doesn't exist anymore, but instead we need an option
to just skip installing the key entirely. This allows it to be used in
labs where we want to install the public cert but not the key.

so just re-using $privatekey and only installing the key if this != "false".

also don't quote the booleans

make the Exec creating the pkcs12 run "onlyif" the key file is not empty

Change-Id: If507d39b0f51ac1de922cc80de89ce244832ba97
Merge "install_certificate: change the behaviour for $privatekey=true/false." into production