The default PCR bank for TPM2 is sha1, which is not always supported (it is legacy and optional for implementation). Make this more future-proof and use the first bank with non-empty set of PCRs, which is returned from TPM by tpm2_getcap pcrs.
The swtpm by default does not create sha1 bank, so this fixes usage with swtpm.

Reproducible: Always

Steps to Reproduce:
1.Configure TPM2 bank without sha1
2.Try configuring clevis without specifying bank
3.Check error:
# echo foo | clevis encrypt tpm2 '{"pcr_ids": "7"}' | clevis decrypt
Unable to validate combination of PCR bank 'sha1' and PCR IDs '7'.

Actual Results:  

# echo foo | clevis encrypt tpm2 '{"pcr_ids": "7"}' | clevis decrypt
Unable to validate combination of PCR bank 'sha1' and PCR IDs '7'.



Expected Results:  
Encryption/Decryption should be performed correctly independently of the bank:
# echo foo | clevis encrypt tpm2 '{"pcr_ids": "7"}' | clevis decrypt
foo