systemd: enforce en_US.UTF-8 locale in systemd units

Python code does detection of the system encoding based on the locale
settings. On RHEL 8.4 development images we somehow get LANG=en_US which
defaults to iso8859-1 _inside_ the systemd-started service, even though
the whole environment defaults to LANG=en_US.UTF-8.

When instrumented with ExecStartPre=/usr/bin/locale, the following
output can be seen:

locale[45481]: LANG=en_US
locale[45481]: LC_CTYPE="en_US"
locale[45481]: LC_NUMERIC="en_US"
locale[45481]: LC_TIME="en_US"
locale[45481]: LC_COLLATE="en_US"
locale[45481]: LC_MONETARY="en_US"
locale[45481]: LC_MESSAGES="en_US"
locale[45481]: LC_PAPER="en_US"
locale[45481]: LC_NAME="en_US"
locale[45481]: LC_ADDRESS="en_US"
locale[45481]: LC_TELEPHONE="en_US"
locale[45481]: LC_MEASUREMENT="en_US"
locale[45481]: LC_IDENTIFICATION="en_US"
locale[45481]: LC_ALL=
ipactl[45483]: Unexpected error
ipactl[45483]: SystemEncodingError: System encoding must be UTF-8, 'iso8859-1' is not supported. Set LC_ALL="C.UTF-8", or LC_ALL="" and LC_CTYPE="C.UTF-8".
systemd[1]: ipa.service: Main process exited, code=exited, status=1/FAILURE

Set the environment to explicit LC_ALL=C.UTF-8 to please the Python
code. FreeIPA server side only cares about actual encoding, not the
language itself. We already use LC_ALL=C.UTF-8 in httpd service snippet.

Fixes: https://pagure.io/freeipa/issue/8617
Signed-off-by: Alexander Bokovoy <abokovoy@redhat.com>
Reviewed-By: Thomas Woerner <twoerner@redhat.com>

ipaserver/install/cainstance.py

@@ -629,6 +629,7 @@ class CAInstance(DogtagInstance):
         with open(conf, 'w') as f:
             os.fchmod(f.fileno(), 0o644)
             f.write('[Service]\n')
+            f.write('Environment=LC_ALL=C.UTF-8\n')
             f.write('ExecStartPost={}\n'.format(paths.IPA_PKI_WAIT_RUNNING))
         tasks.systemd_daemon_reload()