Update config files for LC resources

Changes the config files in CIME to reflect recent changes in Livermore
Computing resources:

Update syrah and quartz machine config entries to explicitly load
mvapich2/2.2 which is the only mvapich version on LC that grants
access to pnetcdf/1.9.0.

These changes reflect a change in the default module loads on LC
machines.

In addition, there is a change to the run_script to purge all mention
of the machines cab and sierra (which have been taken down) and to
add a warning message to the build section warning of the sensitivity
of CIME to double slashes (//) in the case directory path.

[BFB] - Bit-For-Bit

See confluence for a more detailed description about these tags.

config/e3sm/machines/config_machines.xml

@@ -1428,8 +1428,10 @@
       <modules compiler="intel">
         <command name="load">python</command>
         <command name="load">git</command>
-        <command name="load">intel</command>
-        <command name="load">mvapich2</command>
+        <command name="load">intel/18.0.1</command>
+        <command name="unload">pnetcdf/1.9.0</command>
+        <command name="unload">mvapich2</command>
+        <command name="load">mvapich2/2.2</command>
         <command name="load">netcdf-fortran/4.4.4</command>
         <command name="load">pnetcdf/1.9.0</command>
       </modules>
@@ -1478,8 +1480,10 @@
       <modules compiler="intel">
         <command name="load">python</command>
         <command name="load">git</command>
-        <command name="load">intel</command>
-        <command name="load">mvapich2</command>
+        <command name="load">intel/18.0.1</command>
+        <command name="unload">pnetcdf/1.9.0</command>
+        <command name="unload">mvapich2</command>
+        <command name="load">mvapich2/2.2</command>
         <command name="load">netcdf-fortran/4.4.4</command>
         <command name="load">pnetcdf/1.9.0</command>
       </modules>