Description of problem:
When using the IPA server in RHEL 7, Fedora 27 clients fail to fetch vault secrets

[batkisso@dione tmp]$ ipa vault-retrieve iam-vault --shared --out secret.txt
ipa: ERROR: non-public: TypeError: write() argument must be str, not bytes
Traceback (most recent call last):
  File "/usr/lib/python3.6/site-packages/ipalib/backend.py", line 141, in execute
    return self.Command[_name](*args, **options)
  File "/usr/lib/python3.6/site-packages/ipalib/frontend.py", line 450, in __call__
    return self.__do_call(*args, **options)
  File "/usr/lib/python3.6/site-packages/ipalib/frontend.py", line 478, in __do_call
    ret = self.run(*args, **options)
  File "/usr/lib/python3.6/site-packages/ipalib/frontend.py", line 1204, in run
    return self.forward(*args, **options)
  File "/usr/lib/python3.6/site-packages/ipaclient/plugins/vault.py", line 1139, in forward
    f.write(data)
TypeError: write() argument must be str, not bytes
ipa: ERROR: an internal error has occurred


Version-Release number of selected component (if applicable):
freeipa-client-4.6.3-2.fc27.x86_64
freeipa-client-common-4.6.3-2.fc27.noarch
freeipa-common-4.6.3-2.fc27.noarch

server:
ipa-server-4.5.0-22.el7_4.x86_64
ipa-server-dns-4.5.0-22.el7_4.noarch
ipa-common-4.5.0-22.el7_4.noarch
ipa-client-common-4.5.0-22.el7_4.noarch
ipa-client-4.5.0-22.el7_4.x86_64
ipa-server-common-4.5.0-22.el7_4.noarch


This works fine on a RHEL 7.4 client:
[batkisso@aeon tmp]$ ipa vault-retrieve iam-vault --shared --out secret.txt
-------------------------------------
Retrieved data from vault "iam-vault"
-------------------------------------
[batkisso@aeon tmp]$ rpm -qa | egrep '^ipa'
ipa-common-4.5.0-22.el7_4.noarch
ipa-client-common-4.5.0-22.el7_4.noarch
ipa-client-4.5.0-22.el7_4.x86_64