fprintd should enroll a user, not a uid (BugFix) (#813)

fprintd should enroll to a user while not uid. (BugFix) (#813)
$ sudo env | grep SUDO
SUDO_COMMAND=/usr/bin/env
SUDO_USER=ubuntu
SUDO_UID=1001
SUDO_GID=1001

when running checkbox with sudo, it will 'fprintd-enroll 1001'
and the test case will lock screen to let 'ubuntu' user to login,
but at that time, it will not work because the user is not 'ubuntu'.

providers/base/units/fingerprint/jobs.pxu

@@ -3,7 +3,7 @@ category_id: com.canonical.plainbox::fingerprint
 id: fingerprint/detect
 flags: also-after-suspend
 user: root
-command: fprintd-list "${SUDO_UID:-$NORMAL_USER}"
+command: fprintd-list "${SUDO_USER:-$NORMAL_USER}"
 estimated_duration: 1.0
 _summary: Check if at least one fingerprint reader is detected
 imports: from com.canonical.plainbox import manifest
@@ -18,9 +18,9 @@ user: root
 command:
   set -e
   echo 'Removing existing signatures...'
-  fprintd-delete "${SUDO_UID:-$NORMAL_USER}" > /dev/null
+  fprintd-delete "${SUDO_USER:-$NORMAL_USER}" > /dev/null
   sleep 3
-  fprintd-enroll "${SUDO_UID:-$NORMAL_USER}"
+  fprintd-enroll "${SUDO_USER:-$NORMAL_USER}"
 estimated_duration: 20.0
 _summary: Enroll a fingerprint
 _purpose:
@@ -43,7 +43,7 @@ depends: fingerprint/enroll
 user: root
 command:
   sleep 2
-  fprintd-verify "${SUDO_UID:-$NORMAL_USER}" | tee /dev/stderr | grep -o verify-no-match
+  fprintd-verify "${SUDO_USER:-$NORMAL_USER}" | tee /dev/stderr | grep -o verify-no-match
 estimated_duration: 20.0
 _summary: Fingerprint negative match
 _purpose:
@@ -69,7 +69,7 @@ command:
   for i in {1..3}
   do
       # shellcheck disable=SC2015
-      fprintd-verify "${SUDO_UID:-$NORMAL_USER}" | tee /dev/stderr | grep -o verify-match && break || sleep 3
+      fprintd-verify "${SUDO_USER:-$NORMAL_USER}" | tee /dev/stderr | grep -o verify-match && break || sleep 3
       [[ $i -eq 3 ]] && exit 1
       echo "Attempts left: $((3-i))"
   done
@@ -117,8 +117,8 @@ id: fingerprint/delete
 flags: also-after-suspend
 after: fingerprint/unlock
 user: root
-command: fprintd-delete "${SUDO_UID:-$NORMAL_USER}"
+command: fprintd-delete "${SUDO_USER:-$NORMAL_USER}"
 estimated_duration: 1.0
 _summary: Remove existing fingerprint signatures
 imports: from com.canonical.plainbox import manifest
-requires: manifest.has_fingerprint_reader == 'True'
+requires: manifest.has_fingerprint_reader == 'True'