Use SHA256 instead of MD5 for /proc/mounts hash calculation

[vojtechtrefny]

MD5 is not available in the FIPS mode.

Resolves: rhbz#1792340

[dwlehman]

Looks good to me with the assumption that SHA256 will always be available.

[vojtechtrefny]

Looks good to me with the assumption that SHA256 will always be available.

Both SHA1 and SHA256 are currently available in FIPS mode on RHEL 8:

>>> import hashlib
>>> hashlib.md5()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: [digital envelope routines: EVP_DigestInit_ex] disabled for FIPS
>>> hashlib.sha1()
<sha1 HASH object @ 0x7fb7abf409b8>
>>> hashlib.sha256()
<sha256 HASH object @ 0x7fb7abf40a80>

but SHA256 is probably "safer" for the future given recently discovered issues with SHA1.