Add flag to make baremetal.pxe file injection optional

File injection is slow and risky, So make it optional. File injection
was used for getting networking configuration into the baremetal nodes,
but neutron can do this now. File injection is also used for general
file injection and password injection, but those should be optional as
well.

Change-Id: If59641952255e5aaf3767524e2b3c566eefe2b74
Related-Bug: #1178103
Merge "Add flag to make baremetal.pxe file injection optional"