Enable PSI

[cartermckinnon]

Issue #, if available:

Closes #660

Description of changes:

Adds the psi=1 flag to the kernel command line.

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Testing Done

Built a 1.24 AMI, and launched an instance. The pressure fs is mounted:

> cat /proc/pressure/cpu
some avg10=0.01 avg60=0.54 avg300=0.26 total=1072827

> cat /proc/pressure/memory
some avg10=0.00 avg60=0.00 avg300=0.00 total=0
full avg10=0.00 avg60=0.00 avg300=0.00 total=0

> cat /proc/pressure/io
some avg10=1.25 avg60=9.73 avg300=4.58 total=16250491
full avg10=1.24 avg60=9.20 avg300=4.26 total=15030917

[dims]

LGTM