ppg:staging: pin fork servers to the PSP-X.Y.Z release tags

The Percona fork tagged the new releases as PSP-16.15.1 / PSP-17.11.1 /
PSP-18.6.1 instead of the release-X.Y.Z naming used up to 17.10.2 /
18.4.2, so the revision form introduced with PERCONA_PG_PATCH_VERSION
(release-%!{PG_VERSION}.%!{PERCONA_PG_PATCH_VERSION}) keeps resolving
to the release-* *branches* and would never pin itself to a tag.

Switch staging/17, staging/18 and staging/16/tde to
PSP-%!{PG_VERSION}.%!{PERCONA_PG_PATCH_VERSION}.  Each PSP tag
currently points at the exact head of the corresponding release-*
branch (PSP-16.15.1 = c0d38ac6, PSP-17.11.1 = d2798ace,
PSP-18.6.1 = 19c3033d), so the built source is unchanged; the
revision just stops being a moving ref, as a staging tier expects.

Signed-off-by: Ricardo Dias <ricardo.dias@percona.com>

Author: rjd15372

root/ppg/staging/16/tde/percona-postgresql/obs/_service

@@ -2,7 +2,7 @@
   <service name="obs_scm">
     <param name="url">https://github.com/percona/postgres.git</param>
     <param name="scm">git</param>
-    <param name="revision">release-%!{PG_VERSION}.%!{PERCONA_PG_PATCH_VERSION}</param>
+    <param name="revision">PSP-%!{PG_VERSION}.%!{PERCONA_PG_PATCH_VERSION}</param>
     <param name="version">%!{PG_VERSION}</param>
     <param name="filename">percona-postgresql</param>
   </service>

root/ppg/staging/17/percona-postgresql/obs/_service

@@ -2,7 +2,7 @@
   <service name="obs_scm">
     <param name="url">https://github.com/percona/postgres.git</param>
     <param name="scm">git</param>
-    <param name="revision">release-%!{PG_VERSION}.%!{PERCONA_PG_PATCH_VERSION}</param>
+    <param name="revision">PSP-%!{PG_VERSION}.%!{PERCONA_PG_PATCH_VERSION}</param>
     <param name="version">%!{PG_VERSION}</param>
     <param name="filename">percona-postgresql</param>
   </service>

root/ppg/staging/18/percona-postgresql/obs/_service

@@ -2,7 +2,7 @@
   <service name="obs_scm">
     <param name="url">https://github.com/percona/postgres.git</param>
     <param name="scm">git</param>
-    <param name="revision">release-%!{PG_VERSION}.%!{PERCONA_PG_PATCH_VERSION}</param>
+    <param name="revision">PSP-%!{PG_VERSION}.%!{PERCONA_PG_PATCH_VERSION}</param>
     <param name="version">%!{PG_VERSION}</param>
     <param name="filename">percona-postgresql</param>
   </service>