Add --metadataignore to pvcreate.

Add --metadataignore to pvcreate.

Allow metadataignore flag to be passed in to pvcreate.
Ideally, more refactoring of the mda allocation / initialization
is warranted, but for now, we just add another parameter to 'add_mda'
to take an existing mda ignored flag.  We need to do this or pv_write
loses the state of the mda 'ignored' flag before copying and writing
to disk.