Allow user to explicitly specify which CA file is used for verifying
the server certificate by setting 'SSL_CERT_FILE' environment variable.

Note: When this variable is specified, the specified path is the
only CA file which is used to verifying the server certificate.

Part of LIBCLOUD-283.


git-svn-id: https://svn.apache.org/repos/asf/libcloud/trunk@1440289 13f79535-47bb-0310-9956-ffa450edef68