Load legacy openssl provider when buildpack.yml config is set

[sophiewigmore]

If a user sets the following in their buildpack.yml file:

use_legacy_openssl: true

It will enable the legacy openssl provider to be loaded in via adding an openssl.cnf file to the application workspace with the following content:

[provider_sect]
default = default_sect
legacy = legacy_sect

[default_sect]
activate = 1

[legacy_sect]
activate = 1

This ties in with the change made in this PR: cloudfoundry/cflinuxfs4#11, but will add the openssl.cnf file directly to the workspace without a user needing to add this file themselves.

If a user wants to provide their own openssl.cnf file, that will take precedence over this functionality.