(2.9) dcache-webadmin: revert rrd data source names

The following releases

2.10.8
2.9.12
2.8.16
2.7.21
2.6.36

introduced improvements to the pool queue plots code
which also unfortunately altered the data source names
(mainly in order to eliminate NaNs from the underlying
store). This results in an incompatibility with
existing .rrd files preventing plot generation (see
the exception trace below).

While the round-robin datastore is transient (the old
default was 15 days of queue information, now changed to 2)
and can be eliminated and then recreated afresh, this
patch restores the old names, so that upgrades from before
these releases to the current will avoid this issue.
It turns out that the storing of NaNs has no impact
on the principal issue the original patch was trying
to address (faulty stacked rendering of histogram bars).

Those who have already moved to the indicated releases
will see the stack trace again and will have
to remove the .rrd files.  Those upgrading from
a version prior to those indicated will experience
no such error.

Target: master
Patch: https://rb.dcache.org/r/7434
Acked-by: Tigran
Require-book: no
Require-notes: yes

RELEASE NOTES:  Those upgrading to

2.10.8
2.9.12
2.8.16
2.7.21
2.6.36

Will experience an error message regarding the pool-queue plots reporting "Unknown datasource name".
The problem can be fixed in the following ways:

1) No upgrade.  Do:

dcache stop httpdDomain
rm -f /var/lib/dcache/plots/*.rrd
dcache start httpDomain

2) Upgrade to this release.

If a fix such as 1) was applied, and then an upgrade is done, 1) will have to be reapplied.

Upgrading from a version preceding the listed versions to this one should experience no
such error.