Rename FstabEntry::metadata_encryption to metadata_encryption_options

There have been two bugs where people use !metadata_encryption.empty()
to check whether metadata encryption is enabled.  It should actually be
!metadata_key_dir.empty(), since 'metadata_encryption' is the encryption
options, which can be empty if the defaults are sufficient.

Rename the field in FstabEntry appropriately.

To avoid breaking fstab files, don't rename the flag in the fstab file
itself.  So, now the fstab flags map to FstabEntry fields as follows:

    keydirectory => metadata_key_dir
    metadata_encryption => metadata_encryption_options

Change-Id: I5bf673047c99e077bd6e1ac006d80e7e16bc814b