Add hostboot-binaries package env version/site overrides

Support environment override of HOSTBOOT_BINARIES_VERSION 
Support environment override of HOSTBOOT_BINARIES_SITE
Add hostboot package env version/site overrides

Support environment override of HOSTBOOT_VERSION 
Support environment override of HOSTBOOT_SITE
Add ffs package env version/site overrides

Support environment override of OPENPOWER_FFS_VERSION 
Support environment override of OPENPOWER_FFS_SITE
Add pnor package env version/site overrides

Support environment override of OPENPOWER_PNOR_VERSION 
Support environment override of OPENPOWER_PNOR_SITE
Add targeting package env version/site overrides

Support environment override of OPENPOWER_TARGETING_VERSION
Support environment override of OPENPOWER_TARGETING_SITE
Merge pull request #18 from bofferdn/optional-settings-2

Optional settings 2