Bump up default log level, key import/export logs

[riyazdf]

• makes warn the default log level on notary CLI
• increases the -v log level from error to info
• bumps up key import/export failure logs from info to warn

cc @cyc115 @ecordell

Closes #1162, #1178

Ex: here's what an offline notary list looks like now if there's a cache with a valid timestamp:

🐳 $ ./notary -s https://notary.docker.io list docker.io/library/alpine
ERRO[0000] could not reach https://notary.docker.io: Get https://notary.docker.io/v2/: dial tcp: lookup notary.docker.io on [::1]:53: read udp [::1]:56941->[::1]:53: read: connection refused
WARN[0000] Error while downloading remote metadata, using cached timestamp - this might not be the latest version available remotely
NAME               DIGEST                                                              SIZE (BYTES)    ROLE
----               ------                                                              ------------    ----
2.6                9ace551613070689a12857d62c30ef0daa9a376107ec0fff0e34786cedb3399b    528             targets
2.7                9f08005dff552038f0ad2f46b8e65ff3d25641747d3912e3ea8da6785046561a    1374            targets
<..snip..>

[endophage]

LGTM

[ashfall]

🚢

[ecordell]

LGTM

[cyli]

LGTM