apiserver: use systemd-notify(1)

[iliana]

Issue #, if available: n/a

Description of changes:
This avoids use of the systemd and libsystemd-sys crates just for sending the start-up completion message to systemd.

Launched instances with this change and was able to access the API server to enable the admin container and run:

bash-5.0# LD_TRACE_LOADED_OBJECTS=1 /x86_64-thar-linux-gnu/sys-root/usr/lib64/ld-linux-x86-64.so.2 /usr/bin/apiserver 
        linux-vdso.so.1 (0x00007ffeef183000)
        libstd-e79e6f553aa6b60d.so => /x86_64-thar-linux-gnu/sys-root/usr/lib/libstd-e79e6f553aa6b60d.so (0x00007f6cc14ff000)
        libdl.so.2 => /x86_64-thar-linux-gnu/sys-root/usr/lib/libdl.so.2 (0x00007f6cc14fa000)
        librt.so.1 => /x86_64-thar-linux-gnu/sys-root/usr/lib/librt.so.1 (0x00007f6cc14f0000)
        libpthread.so.0 => /x86_64-thar-linux-gnu/sys-root/usr/lib/libpthread.so.0 (0x00007f6cc14ce000)
        libgcc_s.so.1 => /x86_64-thar-linux-gnu/sys-root/usr/lib/libgcc_s.so.1 (0x00007f6cc14b4000)
        libc.so.6 => /x86_64-thar-linux-gnu/sys-root/usr/lib/libc.so.6 (0x00007f6cc132e000)
        /lib64/ld-linux-x86-64.so.2 => /x86_64-thar-linux-gnu/sys-root/usr/lib64/ld-linux-x86-64.so.2 (0x00007f6cc1a3b000)
        libm.so.6 => /x86_64-thar-linux-gnu/sys-root/usr/lib/libm.so.6 (0x00007f6cc1266000)
bash-5.0# systemctl status apiserver
● apiserver.service - Thar API server
     Loaded: loaded (/x86_64-thar-linux-gnu/sys-root/usr/lib/systemd/system/apiserver.service; enabled; vendor preset: enabled)
     Active: active (running) since Tue 2020-01-28 23:19:05 UTC; 5min ago
   Main PID: 2177 (apiserver)
      Tasks: 5 (limit: 9201)
     Memory: 6.0M
     CGroup: /system.slice/apiserver.service
             └─2177 /usr/bin/apiserver --datastore-path /var/lib/thar/datastore/current --socket-gid 274

Jan 28 23:19:05 localhost systemd[1]: Starting Thar API server...
Jan 28 23:19:05 localhost systemd[1]: Started Thar API server.

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[tjkirch]

I tested that this works with a local apiserver.